Polarization-mode dispersion detecting method, and a dispersion compensation controlling apparatus and a dispersion compensation controlling method

ABSTRACT

A dispersion compensation controlling apparatus used in a very high-speed optical communication system adopting optical time division multiplexing system comprises a first specific frequency component detecting unit ( 2   a ) detecting a first specific frequency component in a baseband spectrum in a transmission optical signal inputted to a receiving side over a transmission fiber as a transmission line ( 6   a ), a first intensity detecting unit ( 3   a ) detecting information on an intensity of the first specific frequency component detected by the first specific frequency component detecting unit ( 2   a ), and a polarization-mode dispersion controlling unit ( 220   a ) controlling a polarization-mode dispersion quantity of the transmission line ( 6   a ) such that the intensity of the first specific frequency component detected by the first intensity detecting unit ( 3   a ) becomes the maximum, thereby easily detecting and compensating polarization-mode dispersion generated in a high-speed optical signal.

This continuing application is filed under 35 U.S.C. §111(a), based uponInternational Application PCT/JP98/05336, filed Nov. 27, 1998, it beingfurther noted that priority is based upon Japanese Patent ApplicationHEI 09-328612, filed Nov. 28, 1997.

TECHNICAL FIELD

The present invention relates to a polarization-mode dispersiondetecting method, and a dispersion compensation controlling apparatusand a dispersion compensation controlling method used whenpolarization-mode dispersion or chromatic dispersion of a transmissionoptical signal which becomes a factor of limitation on a transmissiondistance of a high-speed optical signal in a very high-speed opticalcommunication system adopting, for example, optical time divisionmultiplexing.

BACKGROUND ART

In a trunk-line optical communication system, a system with atransmission rate 10 Gb/s (gigabit/second) is in stage of practicalapplication. On the other hand, there is a demand for a larger capacityof the optical communication system with a rapid increase of aninformation quantity. Considered as candidates for employable system aretime division multiplexing (including optical time divisionmultiplexing) and wavelength division multiplexing. Particularly, intime division multiplexing, a lot of researches on a very high-speedoptical communication system with a transmission rate 40 Gb/s(hereinafter referred to as a 40 Gb/s optical communication system) areconducted inside and outside the country.

However, the 40 Gb/s optical communication system has a problem that atransmission distance of an optical signal is limited since atransmission waveform is deteriorated by effects of polarization-modedispersion and chromatic dispersion. Namely, in this system transmissionline, a chromatic dispersion value and a polarization-mode dispersionvalue are factors of limitations of a transmission rate and atransmission distance. Hereinafter, results of simulation and results ofexperiment on chromatic dispersion will be described with reference toFIGS. 66 through 72, and polarization-mode dispersion will be describedwith reference to FIGS. 73 through 75.

Although a term “dispersion” is generally used to mean “chromaticdispersion”, when merely the term “dispersion” is used hereinafter, itmeans both “polarization-mode dispersion” and “chromatic dispersion”unless specifically mentioned.

First, chromatic dispersion will be schematically described. Since achromatic dispersion tolerance (tolerance means an allowance) isinversely proportional to the square of a bit rate, a chromaticdispersion tolerance of 10 Gb/s is 800 ps/nm, while a chromaticdispersion tolerance of 40 Gb/s is about 50 ps/nm that is one sixteenthof 800 ps/nm, which is severer.

FIG. 66 shows a structure of an experimental system to evaluatedispersion compensation tolerance after 50 km transmission over a 1.3 μmzero-dispersion fiber (SMF: Single Mode Fiber) in 40 Gb/s optical timedivision multiplexing (OTDM: Optical Time Division Multiplexing). Hereare used a chromatic dispersion value=18.6 ps/nm/km, and a totaldispersion value=930 ps/nm. A 40 Gb/s optical transmitter 121 a shown inFIG. 66 is a signal light source. A signal light intensity-modulated inan intensity modulator 121 b is inputted to a receiving side(hereinafter referred to as a receiving terminal, occasionally) over aDCF (Dispersion Compensating Fibers) 124 via the SMF 123. On thereceiving side, a preamplifier 122 a and a 40 Gb/s optical receiver 122b perform a demodulating process.

FIG. 67 shows a result of an evaluation experiment in this experimentalsystem, wherein a transverse axis represents total dispersion quantity(unit: ps/nm) while a vertical axis represents power penalty (unit: dB).If here is required a power penalty 1 dB or less as an evaluationreference of the transmission line, a dispersion compensation tolerance(dispersion width) is 30 ps/nm, this value corresponding to 2 km or lessin transmission using SMF. Namely, when a repeater spacing, that is, adistance between stations, is not constant as in a ground system, it isnecessary to optimize a dispersion compensation quantity (high-accuracydispersion compensation of about 100%) of each repeater section.

Additionally, a chromatic dispersion value of an optical fibertransmission line changes with time with a change of laying environmentsuch as temperature, pressure and the like. For example, in the case ofa change in temperature from −50 to 100° C., a quantity of the change indispersion of SMF 50 km is estimated to be 16 ps/nm as shown by thefollowing formula: $\begin{matrix}{\left\lbrack {{dispersion}\quad {change}\quad {quantity}} \right\rbrack = \quad {\left\lbrack {{temperature}\quad {dependency}\quad {of}\quad {zero}\quad {dispersion}\quad {wavelength}} \right\rbrack \times}} \\{\quad {\left\lbrack {{temperature}\quad {change}} \right\rbrack \times \left\lbrack {{dispersion}\quad {slope}} \right\rbrack \times}} \\{\quad \left\lbrack {{transmission}\quad {distance}} \right\rbrack} \\{= \quad {0.03\quad \left( {{{nm}/{^\circ}}\quad {C.}} \right) \times 150\quad \left( {{^\circ}\quad {C.}} \right) \times 0.07\quad \left( {{{ps}/{nm}^{2}}/{km}} \right) \times 50\quad ({km})}} \\{= \quad {16\quad {{ps}/{nm}}}}\end{matrix}$

This value is more than a half of the dispersion tolerance 30 ps/nm,which has to be considered in full in system designing. The reason isthat when a temperature becomes 100° C. during system operation, hevalue does not meet the reference of penalty 1 dB in the worst case,even if the dispersion compensation quantity is optimized at −50° C.when the operation of the system is started. Depending oncharacteristics or a structure of the dispersion compensator, it isimpossible to continuously set a dispersion compensation quantity, sothat there is a case where the dispersion compensation quantity can beset to only a value slightly deviated from an optimum value when theoperation of the system is started. In this case, the value might notmeet the reference of penalty 1 dB even with a change in temperaturebelow 150° C.

In the above consideration, in order to realize a very high-speedoptical communication system above 40 Gb/s, it is necessary to firstoptimize dispersion equalization (dispersion compensation quantity) ineach repeater section when the system operation is started, and tosecondary configure “an automatic dispersion equalization (compensation)system” optimizing dispersion equalization (dispersion compensationvalue) correspondingly to a change with time of a transmission linedispersion value even during the system operation. Meanwhile, thisautomatic dispersion equalization system is required not only in the SMFtransmission system but also in the case where a 1.55 μm wavelengthdispersion shifted fiber (DSF: Dispersion Shifted Fiber) having a smallchromatic dispersion value is used. Elemental techniques for realizingthe automatic dispersion equalization system are summarized into threepoints, (a) through (c) below:

(a) realization of a variable dispersion equalizer (compensator);

(b) method of monitoring a chromatic dispersion value (or a totaldispersion quantity after dispersion equalization [compensation]) of atransmission line; and

(c) method of controlling feedback optimization of a variable dispersionequalizer (compensator).

As a method of measuring a chromatic dispersion value of an opticalfiber, there has been used a pulse method or a phase method in whichlight having plural different wavelengths is inputted to an opticalfiber, and a group delay difference or a phase difference in the outputlight is measured. However, in order to always measure dispersion duringthe system operation using these methods, a set of chormatic dispersionmeasuring devices are required in each repeater section. Further, inorder to measure a dispersion quantity without interrupting transmissionof data signal light, it is necessary to wavelength-multiplex measuringlight having a wavelength different from that of the data signal light.

Assembling the pulse method or the phase method in an opticaltransmission apparatus is not realistic from the points of view of sizeand economy. Further, when a wavelength different from that of the mainsignal light, there is a possibility of lacking accuracy since it isnecessary to perform a process to assume a dispersion value at awavelength of the signal light from a measured value at a wavelength ofthe measuring light. For this, a method being able to directly monitor awavelength dispersion value from the main signal light is desirable.

As this wavelength dispersion monitoring method, there has been alreadyproposed in the Conferrence and the like a method using a 40 GHzcomponent intensity in a baseband spectrum of a 40 Gb/s OTDM signal andan NRZ (Non-Return-to-Zero) signal.

FIG. 68 shows a relationship (simulation results) between 40 GHzcomponent intensity and eye-opening with respect to dispersion quantityof a 40 Gb/s OTDM signal. Between two curves shown in FIG. 68, onehaving a pair of peaks represents 40 GHz component intensity, while theother one having a single peak represents eye-opening, wherein theminimum point between the pair of peaks of the 40 GHz componentintensity is zero dispersion point, at which the eye-opening is themaximum.

FIG. 69 shows a structure of an experimental system at the time of DSF100 km transmission. Signal light is sent from a transmitting side(hereinafter referred to as a transmitting terminal, occasionally) 131shown in FIG. 69, and a temperature of a fiber that is a transmissionline can be changed in a thermostat 133. On a receiving side 132, a 40GHz component intensity is measured.

FIG. 70 shows results of the experiment in the experimental system,wherein a transverse axis represents signal light wavelength, while avertical axis represents monitor voltage at a 40 GHz componentintensity. The signal light wavelength that is the transverse axis isswept in a range from 1535 to 1565 nm [nanometer: (nano represents theminus ninth power of 10)], while the monitor voltage represents resultsat three kinds of temperatures. In each of these three kinds ofwaveforms, the minimum point between a pair of peaks of the waveformshows zero dispersion wavelength, like the simulation result shown inFIG. 68. Following a change in temperature (−35 to +65° C.) of DSF 100km, it is known that the zero dispersion wavelength is changed (0.027nm/° C.).

FIG. 71(a) shows a relationship (simulation results) between 40 GHzcomponent intensity and eye-opening with respect to a dispersionquantity of a 40 Gb/s NRZ signal (α=−0.7). In FIG. 71(a), one having aplurality of peaks represents 40 GHz component intensity, while theother one having a single peak represents eye-opening, as well. In thecase of α<0, the 40 Gb/s component intensity has the maximum peak in thevicinity of +30 ps/nm, and the monitor value shows zero that is theminimum value in zero dispersion at the foot on the negativedispersion's side.

FIG. 71(b) shows results of an experiment at the time of DSF 100 kmtransmission when a temperature is changed from −35 to +65° C. As wellas the simulation results [refer to FIG. 71(a)], the minimum value atthe foot on the long wavelength's side of the maximum peak [refer to apoint denoted by 134 in FIG. 71(b)] shows zero dispersion wavelength,and the zero dispersion wavelength is changed at 0.026 nm/°C., whichcoincides with the results in FIG. 70. FIG. 71(a) shows simulationresults in the case of a 40 Gb/s NRZ signal (α=+0.7). FIG. 72(b) showssimulation results in the case of a 40 Gb/s RZ (Return-to-Zero) signal(α=0, Duty=50%). In such the automatic dispersion compensation system,it is necessary to feedback-control an operation point of a variabledispersion (equalization) compensator such that the eye-opening becomesthe maximum using the above chromatic dispersion monitor.

Next, polarization-mode dispersion (PMD: Polarization-Mode Dispersion)that is the second factor having an effect on a transmission distance inthe 40 Gb/s system will be schematically described. Polarization-modedispersion (PMD) is caused by that propagation delay times ofpolarization components (light in two modes such as TE mode and TM mode,for example) of a light signal are different, which might generate inany optical fiber. Generally, the larger a transmission rate of anoptical signal or the longer a transmission distance of an opticalsignal, the larger is an effect of polarization-mode dispersion, whichcannot be ignored. It is said that some optical fibers configuring oldoptical transmission lines laid mainly in countries other than Japanhave a large PMD value above 1 ps/km^(1/2) [picosecond/km^(1/2) (picorepresents minus twelfth power of 10) per unit length. In the case of ashort distance transmission (for example, 50 km transmission) using suchoptical fibers, an optical delay difference (Δτ) is 7 ps or larger perone time slot 25 ps of 40 Gb/s, where an effect of polarization-modedispersion cannot be ignored. Incidentally, this value is determinedaccording to a type of optical fiber, which does not depend on atransmission rate of an optical signal. Further, since it is practicallynecessary to provide devices generating polarization-mode dispersionsuch as an optical amplifier, a wavelength dispersion compensator andthe like in an optical communication system, a transmission distance ofan optical signal is further limited.

Accordingly, in order to increase a transmission rate of an opticalsignal while still using an optical transmission line having beenalready laid or perform long-distance in-line repeater transmissionwhile still using an optical transmission line having been already laid,a technique of compensating polarization-mode dispersion generated in atransmit optical signal is demanded.

As methods of compensating polarization-mode dispersion, there arecompensating methods described in publications shown below, for example.Incidentally, it is difficult to thoroughly compensate transmit waveformdeterioration since mode coupling due to fluctuation of birefringence ina longitudinal direction of an optical fiber is complicatedly generatedeven with an optical fiber configuring an actual optical transmissionline, moreover, the mode coupling is changed with time due totemperature change and the like. In order to relieve transmissionwaveform deterioration, methods described in publications {circle around(1)} through {circle around (3)} shown below are effective.

{circle around (1)} Method of providing a polarization controller (PC:Polarization Controller) at a transmitting terminal of an opticalsignal, feeding back transmission characteristic from the receivingterminal so as to control a splitting ratio γ of an optical intensity totwo polarization modes to be 0 or 1 (J. H. Winters et al., “Opticalequalization of polarization dispersion”, SPIE Vol.1787 MultigigabitFiber Communications, 1992, pp.346-357).

{circle around (2)} Method of providing a polarization controller and apolarization maintaining fiber (PMF: Polarization Maintaining Fiber) ata receiving terminal of an optical signal, and controlling thepolarization controller to give a delay difference (fixed value) betweentwo polarization modes of an inverse code to an optical transmissionline (T. Takahashi et al., “Automatic compensation technique fortimewise fluctuating polarization-mode dispersion in in-line amplifiersystems”, Electro.Lett., vol.30, No.4, 1994, pp.348-349); and

{circle around (3)} Method of providing a polarization controller, apolarization beam splitter (PBS: Polarization Beam Splitter), photoreceivers receiving two optical signal components split by thepolarization beam splitter, and a variable delay element giving a delaydifference between two electric signals obtained by the photo receiversto control the polarization controller and the variable delay element(T. Ono et al., “Polarization Control Method for SuppressingPolarization-mode Dispersion Influence in Optical Transmission Systems”,J. of Lightwave Tecnol., vol.12, no.5, 1994, pp.891-898).

In any of these methods {circle around (1)} through {circle around (3)},it is necessary to detect a state of polarization-mode dispersion at areceiving terminal of an optical signal to perform a feed-back control.However, there is required not a complicated method using a result ofdetection of a code error rate or the like but a technique of easilydetecting a state of polarization-mode dispersion. Such an opticalcommunication systems will be required in future that a bit rate, atransmission distance, a signal modulation format and the like can befreely changed. For this, even in a technique of compensatingpolarization-mode dispersion, it is required to comply with fluctuationsof a state of polarization-mode dispersion generated in a transmissionline.

FIG. 73 shows an experimental system for studying transmission waveformdeterioration of a 40 Gb/s signal by PMD. An optical intensity splittingratio (or an optical power ratio) γ of each polarization component ofsignal light sent out from a transmitting side 133 shown in FIG. 73 ischanged in a polarization controller 134, the signal light is added PMDgenerated in a transmission line in a PMD emulator (PMD emulator) 135and demodulated in a receiving terminal 136. The PMD emulator 135simulates PMD generated in the transmission line, wherein a commerciallyavailable PMD emulator is used. Principles upon which the PMD emulator135 operates are as follows. Namely, the signal light is split into twopolarization components by the polarization beam splitter (PBS) 135 ashown in FIG. 73, one of which is given an optical delay difference Δτ(ps) in an optical delay device 135 b, the other of which is given aloss in an optical attenuator 135 such that optical losses in the bothoptical paths are equal. Further, they are multiplexed while they arestill in an orthogonal state by a polarization beam splitter (PBS) 135d. The output signal is amplified by an optical preamplifier 136 a inthe receiving terminal 136, and demodulated in an optical DEMUX(Demultiplex) 136 b.

FIG. 74 shows results of an experiment of evaluation of power penalty tooptical delay difference Δτ of a 40 Gb/s OTDM signal and a NRZ signal.The transversal axis represents optical delay difference Δτ, while avertical axis represents power penalty. Incidentally, γ is set to 0.5 inthe polarization controller 134 (refer to FIG. 73) such thattransmission waveform deterioration is the maximum. A curved linedenoted by (a) in FIG. 74 represents transmission waveform deteriorationof the OTDM signal. When a reference value of receiver sensitivitydegradation (power penalty [vertical axis]) is below 1 dB, a PMDallowable value (PMD tolerance) is 9 ps. A curved line denoted by (b) inFIG. 74 represents transmission waveform deterioration of the 40 Gb/sNRZ signal. When a reference value of receiver sensitivity degradationat this time is below 1 dB, the PMD allowable value (PMD tolerance) is11 ps.

In consideration of a value of the receiver sensitivity degradation,some relatively old fibers having been already laid have a large PMDvalue above 1.0 ps/km^(1/2) per unit length. In such case, a value ofthe receiver sensitivity degradation is above 10 ps even in a relativelyshort distance transmission of 100 km or less. Further, sincepolarization-mode dispersion is generated even in an optical amplifier,a chromatic dispersion compensator and the like other than atransmission line fiber in an actual optical transmission system, atransmission distance is further limited. In order to increase atransmission distance in a fiber transmission line having been alreadylaid, or in order to perform long-distance in-line repeatertransmission, “PMD compensating technique” is required. However, thiscompensating technique has three problems (d) through (f) below.

(d) realization of a PMD compensating device;

(e) method of detecting a PMD state (optical delay difference Δτ andoptical intensity splitting ratio γ); and

(f) method of controlling feedback-optimization of a PMD compensatingdevice.

Although a PMD measuring device has been commercially available,introducing such PMD measuring device as a part of an opticaltransmission system is not realistic in the view of size and economy. Amethod being able to directly monitor a PMD value is desirable. As suchmethod, there is a method using a frequency component intensity in abaseband spectrum of a received signal, which is theoreticallydetermined as below.

Assuming that F(t) is a change of an optical intensity with time whenPMD is not given, a change of an optical intensity with time when PMD(optical delay difference Δτ and optical density splitting ratio γ) isgiven by the following formula:

γF(t−Δτ)+(1−γ)F(t)

An electric field intensity of an electric signal having been receivedis proportional to its value, and the square of the value is detected asa change of the intensity with time by the intensity detector. Basebandspectrum P(f) is expressed as its Fourier transform by the followingformula (11): $\begin{matrix}\begin{matrix}{{P(f)} = \quad {{\int{{\left\{ {{\gamma \quad {F\left( {t - {\Delta\tau}} \right)}} + {\left( {1 - \gamma} \right){F(t)}}} \right\} \cdot {\exp \left( {{\omega}\quad t} \right)}}{t}}}}^{2}} \\{= \quad {{{\gamma {\int{{F\left( {t - {\Delta\tau}} \right)}{\exp \left( {{\omega}\quad t} \right)}{\quad t}}}} + {\left( {1 - \gamma} \right){\int{{F(t)}{\exp \left( {{\omega}\quad t} \right)}{t}}}}}}^{2}} \\{= \quad {{{{{\gamma exp}({\omega\Delta\tau})}{\int{{F(t)}{\exp \left( {{\omega}\quad t} \right)}{t}}}} + {\left( {1 - \gamma} \right){\int{{F(t)}{\exp \left( {{\omega}\quad t} \right)}{t}}}}}}^{2}} \\{= \quad {{K(f)} \cdot {{\int{{F(t)}{\exp \left( {{\omega}\quad t} \right)}{t}}}}^{2}}}\end{matrix} & (11) \\{{{wherein}\quad a\quad {factor}\quad {of}\quad {proportionality}\quad {K(f)}\quad {is}\quad {expressed}\quad {as}\quad {below}},{{{and}\quad \omega} = {2\pi \quad {f.}}}} & \quad \\\begin{matrix}{{K(f)} = \quad {{{{\gamma exp}({\omega\Delta\tau})} + \left( {1 - \gamma} \right)}}^{2}} \\{= \quad {{{\gamma \left\{ {{\cos ({\omega\Delta\tau})} + {i\quad {\sin ({\omega\Delta\tau})}}} \right\}} + \left( {1 - \gamma} \right)}}^{2}} \\{= \quad {1 - {4{\gamma \left( {1 - \gamma} \right)}{\sin^{2}\left( {\pi \quad f\quad {\Delta\tau}} \right)}}}}\end{matrix} & (12)\end{matrix}$

In formula (11), parameters (optical delay difference Δτ and opticalintensity splitting ratio γ) relating to a PMD state are included inonly K(f), and separated from the baseband spectrum |∫F(t)exp(iωt)dt|²in the case of no PMD. When a frequency component f=fe(Hz) is extractedby a filter or the like and an intensity thereof is detected, dependencyon optical delay difference Δτ and the optical intensity splitting ratioγ is expressed by K(fe). Moreover, from that the formula (11) isestablished for a general formula F(t) representing an optical waveform,the above result that the PMD state can be detected with K(fe) isestablished irrespective of a modulating system (NRZ or RZ) or awaveform change due to such as wavelength dispersion, nonlinear effector the like.

FIG. 75 shows a result of an experiment showing Δτ dependency of 20 GHzcomponents intensity in a 40 Gb/s NRZ system in the case of γ=0.5. Inthis intensity detecting method, an optical signal is converted into anelectric signal using a photo receiver (PD) in the receiving terminal, asignal of a 20 GHz component is extracted by a 20 GHz narrow-bandband-pass filter (BPF), and an intensity is detected by a power meter.As shown in FIG. 75, the intensity is the maximum at optical delaydifference Δτ=0 ps, decreases with increasing the optical delaydifference Δτ, and becomes zero at the optical delay difference Δτ=25ps.

Using that the fe (Hz) component intensity is the maximum when the PMDstate is the best, a method of feedback-controlling thepolarization-mode dispersion compensator controlling the optical delaydifference Δτ and the optical intensity splitting ratio γ inserted inthe optical transmission line (transmitting terminal, optical repeaterand receiving terminal) according to a PMD monitor signal is possible.

Incidentally, there are publications relating to equalization as shownin {circle around (4)} through {circle around (6)} below.

{circle around (4)} publications relating to variable dispersion(equalization) compensator:

R. I. Laming et al., “A Dispersion Tunable Grating in a 10-Gb/s100-200-km-Step IndexFibe Link”, IEEE Photon. Technol. Lett.,vol.8.,pp.428-430,1996. (being able to vary a dispersion compensationquantity by changing a temperature slope in a longitudinal direction ofa chirped fiber grating);

M. M. Ohm et al., “Tunable fiber grating dispersion using apiezoelectric stack”, OFC'97 WJ3. (being able to vary a dispersioncompensation quantity by changing a stress in a longitudinal directionof a chirped fiber grating);

K. Takiguchi et al., “Planar Lightwave Circuit Optical DispersionEqualizer”, IEEE Photon. Technol., Lett., vol.6,no.1,pp.86-88 (PLCvariable dispersion compensator);

A. Sano et al., “Automatic dispersion equalization by monitoringextracted-clockpower level In a 40-Gbit/s, 200-km transmission line”ECOC'96 TuD.3.5 (discreet variable dispersion compensator in whichfibers having a positive or negative dispersion value arecascade-connected by a 1×4 mechanical switch);

{circle around (5)} publications relating to automatic dispersionequalizing system:

G. Ishikawa and H. Ooi, “Demonstration of automatic dispersionequalization in 40-Gbit/s OTDM transmission,” ECOC'98 WdCO6. (introducedin Sep. 23, 1998);

Ooi, Akiyama and Ishikawa, “Experiment on 40 Gbit/s automatic dispersionequalization using a wavelength tunable laser”, EIC. Soc., 1998(Introduced in Sep. 30, 1998);

M. Tomizawa et al., “Automatic Dispersion Equalization for InstallingHigh-Speed Optical Transmission Systems”, J. Lightwave Technol., vol.16,no.2, pp.184-191;

{circle around (6)} publications relating to automatic PMD compensatingsystem:

H.Ooi, Y.Akiyama, G.Ishikawa, “Automatic polarization-mode dispersioncompensation in 40-Gbit/s transmission” (tentative title), submitted toOFC'99 (method of using a polarization controller(PC: PolarizationController) and a polarization maintaining fiber (PMF: PolarizationMaintaining Fiber) in a receiving terminal to control PC in a 40 Gb/sNRZ system, thereby giving a delay difference of an inverse code to atransmission line);

J. H. Winters et al., “optical equalization of polarization dispersion”,SPIE Vol.1787 Multigigabit Fiber Communications, 1992 00.346-357 (methodof using a polarization controller in a transmitting terminal,feeding-back the transmission characteristic from a receiving terminalto control in such a direction as γ=0 or 1)

T. Takahashi et al., “Automatic compensation technique for timewisefluctuating polarization-mode dispersion in in-line amplifier systems”,Electron. Lett., vol.30, no.4, 1994, pp.348-349 (method of giving adelay difference of an inverse code to a transmission line by using apolarization controller (PC) and a polarization maintaining fiber (PMF)in a receiving terminal to control PC), wherein a 5 GHz componentintensity in a baseband spectrum of a 10 Gb/s NRZ signal is detected anda control is performed such that the intensity becomes the maximum;

T. Ono et al., “Polarization Control Method for SuppressingPolarization-mode Dispersion Influence in Optical Transmission Systems”,J. Lightwave Technol., vol.12, no.5, 1994, pp.891-898 (method of using apolarization controller, a polarization beam splitter, photo receiversfor respective light paths and a variable delay element giving a delaydifference between both electric signals to control the PC and thevariable delay element).

In the light of the above problems, an object of the present inventionis to provide a polarization-mode dispersion quantity detecting methodin which polarization-mode dispersion generated in a high-speed opticalsignal can be easily detected and monitored, a dispersion compensationcontrolling method in which these detected polarization-mode dispersionand chromatic dispersion can be compensated, thereby enabling along-distance transmission of a high-speed optical signal, and adispersion compensation controlling apparatus for simultaneouslycompensating transmission optical waveform deterioration caused therebyusing the polarization-mode dispersion quantity detecting method and thechromatic dispersion detecting method.

DISCLOSURE OF INVENTION

Therefore, a dispersion compensation controlling apparatus of thisinvention comprises a first specific frequency component detecting unitfor detecting a first specific frequency component in a basebandspectrum in a transmission optical signal inputted to a receiving sideover a transmission fiber as a transmission line, a first intensitydetecting unit for detecting information on an intensity of the firstspecific frequency component detected by the first specific frequencycomponent detecting unit, and a polarization-mode dispersion controllingunit for controlling a polarization-mode dispersion quantity of thetransmission line such that the intensity of the first specificfrequency component detected by the first intensity detecting unitbecomes the maximum.

Accordingly, it is thereby possible to compensate polarization-modedispersion so as to prevent deterioration of a transmission waveform ofan optical signal. This advantageously contributes to long-distancetransmission of a high-speed optical signal.

Further, a dispersion compensation controlling apparatus of thisinvention comprises a first specific frequency component detecting unitfor detecting a first specific frequency component in a basebandspectrum in a transmission optical signal inputted to a receiving sideover a transmission fiber as a transmission line, a first intensitydetecting unit for detecting information on an intensity of the firstspecific frequency component detected by the first specific frequencycomponent detecting unit, a polarization-mode dispersion controllingunit for controlling a polarization-mode dispersion quantity of thetransmission line such that the intensity of the first specificfrequency component detected by the first intensity detecting unitbecomes the maximum, a second specific frequency component detectingunit for detecting a second specific frequency component in the basebandspectrum in the transmission optical signal, a second intensitydetecting unit for detecting information on the intensity of the secondspecific frequency component detected by the second specific frequencycomponent detecting unit, and a chromatic dispersion controlling unitfor controlling a chromatic dispersion quantity of the transmission linesuch that the intensity of the second specific frequency componentdetected by the second specific frequency intensity detecting unitbecomes the maximum.

Accordingly, it is thereby possible to compensate polarization-modedispersion to prevent deterioration of a transmission waveform of anoptical signal. It is also possible to compensate chromatic dispersionof a transmission optical signal, so as to prevent deterioration of thetransmission waveform of the optical signal by effects ofpolarization-mode dispersion and chromatic dispersion. This moreadvantageously contributes to long-distance transmission of a high-speedoptical signal.

Still further, a dispersion compensation controlling apparatus of thisinvention comprises a first specific frequency component detecting unitfor detecting a first specific frequency component in a basebandspectrum in a transmission optical signal inputted to a receiving sideover a transmission fiber as a transmission line, a first intensitydetecting unit for detecting information on an intensity of the firstspecific frequency component detected by the first specific frequencycomponent detecting unit, a polarization-mode dispersion controllingunit for controlling a polarization-mode dispersion quantity of thetransmission line such that the intensity of the first specificfrequency component detected by the first intensity detecting unitbecomes the maximum, and a chromatic dispersion controlling unit forcontrolling a chromatic dispersion quantity of the transmission linesuch that the intensity of the first specific frequency componentdetected by the first intensity detecting unit becomes the maximum orthe minimum.

Accordingly, it is thereby possible to compensate polarization-modedispersion to prevent deterioration of a transmission waveform of anoptical signal. It is also possible to compensate chromatic dispersionof a transmission optical signal, so as to prevent deterioration of thetransmission waveform of the optical signal by effects ofpolarization-mode dispersion and chromatic dispersion. This moreadvantageously contributes to long-distance transmission of a high-speedoptical signal.

A polarization-mode dispersion quantity detecting method of thisinvention comprises the steps of a specific frequency componentdetecting step of detecting a specific frequency component in a basebandspectrum in a transmission optical signal inputted over a transmissionoptical fiber, an intensity detecting step of detecting an intensity ofthe specific frequency component detected at the specific frequencycomponent detecting step, and a dispersion quantity detecting step ofdetecting a polarization-mode dispersion quantity of the transmissionoptical signal from information on the intensity of the specificfrequency component detected at said intensity detecting step byperforming a predetermined functional operation.

Accordingly, it is thereby possible to easily detect polarization-modedispersion generated in a transmission optical signal.

In addition, a dispersion compensation controlling method of thisinvention comprises the steps of a first specific frequency componentdetecting step of detecting a first specific frequency component in abaseband spectrum in a transmission optical signal inputted to areceiving side over a transmission fiber as a transmission line, a firstintensity detecting step of detecting information on an intensity of thefirst specific frequency component detected at the first specificfrequency component detecting step, and a polarization-mode dispersioncontrolling step of controlling a polarization-mode dispersion quantityof the transmission line such that the intensity of the first specificfrequency component detected at the first intensity detecting stepbecomes the maximum.

Accordingly, it is thereby possible to easily detected polarization-modedispersion generated in a transmission optical signal.

Further, a dispersion compensation controlling method comprises thesteps of a first specific frequency component detecting step ofdetecting a first specific frequency component in a baseband spectrum ina transmission optical signal inputted to a receiving side over atransmission fiber as a transmission line, a first intensity detectingstep of detecting information on an intensity of the first specificfrequency component detected at the first specific frequency componentdetecting step, a polarization-mode dispersion controlling step ofcontrolling a polarization-mode dispersion quantity of the transmissionline such that the intensity of the first specific frequency componentdetected at the first intensity detecting step becomes the maximum, asecond specific frequency component detecting step of detecting a secondspecific frequency component in the baseband spectrum in thetransmission optical signal, a second intensity detecting step ofdetecting information on an intensity of the second specific frequencycomponent detected at the second specific frequency component detectingstep, and a chromatic dispersion controlling step of controlling achromatic dispersion quantity of the transmission line such that theintensity of the second specific frequency component detected at thesecond intensity detecting step becomes the maximum or the minimum.

Accordingly, it is thereby possible to perform the controlsindependently and simultaneously.

Still further, a dispersion compensation controlling method of thisinvention comprises the steps of a first specific frequency componentdetecting step of detecting a first specific frequency component in abaseband spectrum in a transmission optical signal inputted to areceiving side over a transmission fiber as a transmission line, a firstintensity detecting step of detecting information on an intensity of thefirst specific frequency component detected at the first specificfrequency component detecting step, a polarization-mode dispersioncontrolling step of controlling a polarization-mode dispersion quantityof said transmission line such that the intensity of the first specificfrequency component detected at the first intensity detecting stepbecomes the maximum, and a chromatic dispersion controlling step ofcontrolling a chromatic dispersion quantity of the transmission linesuch that the intensity of the first specific frequency componentdetected at the first intensity detecting step becomes the maximum orthe minimum.

Accordingly, it is thereby possible to prevent deterioration of atransmission waveform of an optical signal by effects ofpolarization-mode dispersion and chromatic dispersion, which furthercontributes to long-distance transmission of a high-speed opticalsignal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a structure of a first basic block of thisinvention;

FIG. 2 is a diagram showing a structure of a second basic block of thisinvention;

FIG. 3 is a diagram showing a structure of a third basic block of thisinvention;

FIG. 4 is a block diagram showing a structure of an optical transmissionsystem to which dispersion compensation controlling apparatus accordingto a first embodiment of this invention is applied;

FIG. 5 is a diagram showing a structure of a delay quantity compensator;

FIG. 6 is a diagram showing a structure of an experimental system of a40 Gb/s optical time division multiplexing transmission system accordingto the first embodiment of this invention;

FIG. 7 is a diagram showing a structure of a PMD emulator;

FIGS. 8(a) through 8(e) are diagrams showing deteriorated 40 Gb/soptical time division multiplexed waveforms when the PMD emulatorchanges an optical delay difference Δτ and gives it thereto;

FIG. 9 is a diagram for illustrating a method of detecting apolarization-mode dispersion quantity generated in a transmissionoptical signal;

FIGS. 10(a) and 10(b) are diagrams for illustrating a method ofdetecting a polarization-mode dispersion quantity generated in atransmission optical signal;

FIG. 11 is a diagram showing a structure of an experimental system of a10 Gb/s NRZ transmission system according to the first embodiment ofthis invention;

FIGS. 12(a) through 12(j) are diagrams showing deteriorated 10 Gb/s NRZwaveforms at a receiving terminal when the PMD emulator changes anoptical delay difference Δτ and gives it thereto;

FIG. 13 is a diagram for illustrating a method of detecting apolarization-mode dispersion quantity generated in a transmissionoptical signal;

FIGS. 14(a) and 14(b) are diagrams for illustrating a method ofdetecting a polarization-mode dispersion quantity generated in atransmission optical signal;

FIG. 15 is a diagram showing a structure of an optical time divisionmultiplex modulator;

FIGS. 16(a) through 16(c) are diagrams for illustrating an operatingprinciple of an TODM modulator;

FIG. 17 is a block diagram showing a structure of an opticaltransmission system provided with a dispersion compensation controllingapparatus with a timing extracting unit according to the firstembodiment of this invention;

FIG. 18 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a first modification of the first embodiment ofthis invention is applied;

FIG. 19 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a second modification of the first embodiment ofthis invention is applied;

FIG. 20 is a block diagram showing a structure of another opticaltransmission system to which a dispersion compensation controllingapparatus according to the second modification of the first embodimentof this invention is applied;

FIG. 21 is a block diagram showing a structure of an opticaltransmission system to which another dispersion compensation controllingapparatus according to the second modification of the first embodimentof this invention is applied;

FIG. 22 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a third modification of the first embodiment ofthis invention is applied;

FIG. 23 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a fourth modification of the first embodiment ofthis invention is applied;

FIG. 24 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a fifth modification of the first embodiment ofthis invention is applied;

FIGS. 25(a) through 25(c) are diagrams for illustrating a principle of afeedback control by a compensation quantity optimization controllingunit;

FIGS. 26(a) through 26(g) are diagrams for illustrating an operation ina dispersion compensation controlling apparatus;

FIG. 27 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a sixth modification of the first embodiment ofthis invention is applied;

FIG. 28 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a seventh modification of the first embodiment ofthis invention is applied;

FIG. 29 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to an eighth modification of the first embodiment ofthis invention is applied;

FIG. 30 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a ninth modification of the first embodiment ofthis invention is applied;

FIGS. 31(a) and 31(b) are diagrams showing a change in intensity of aspecific frequency component when parameters showing a polarization-modedispersion quantity to be given to an optical signal by apolarization-mode dispersion compensator undergoes a sweep control in awide range;

FIG. 32 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a second embodiment of this invention is applied;

FIG. 33 is an enlarged diagram of a polarization controller and aninter-polarization-mode variable delay;

FIGS. 34(a) through 34(c) are diagrams showing an example of a variableoptical delay path according to the second embodiment of this invention;

FIG. 35 is a diagram showing an example of a structure of anotherinter-polarization-mode variable delay element according to the secondembodiment of this invention;

FIGS. 36 and 37 are control flowcharts for realizing PMD compensationaccording to the second embodiment of this invention;

FIG. 38 is another control flowchart for realizing the PMD compensationaccording to the second embodiment of this invention;

FIG. 39 is a block diagram showing a structure of an opticaltransmission system according to a second modification of the secondembodiment of this invention;

FIG. 40 is a block diagram showing a structure of an opticaltransmission system according to a third modification of the secondembodiment of this invention;

FIG. 41 is a block diagram showing a structure of an opticaltransmission system to which a PMD compensation controlling apparatus atthe time of system operation according to a fourth modification of thesecond embodiment of this invention is applied;

FIG. 42 is a diagram illustrating a method of measuring a PMD tolerance;

FIG. 43 is a block diagram showing a structure of an opticaltransmission system to which a PMD compensation controlling apparatususing a PMF for PMD compensation according to the fourth modification ofthe second embodiment of this invention is applied;

FIG. 44(a) is a diagram showing a 20 GHz component intensity in areceived baseband signal to α and β;

FIG. 44(b) is a diagram showing an eye opening of a received waveform inthe received baseband signal to α and β;

FIG. 45(a) is a diagram showing a 20 GHz component intensity in areceived baseband signal to α and β;

FIG. 45(b) is a diagram showing an eye opening of a received waveform inthe received baseband signal to α and β;

FIG. 46(a) is a diagram showing a 20 GHz component intensity in areceived baseband signal to α and β;

FIG. 46(b) is a diagram showing an eye opening of a received waveform inthe received baseband signal to α and β;

FIG. 47(a) is a diagram showing a 20 GHz component intensity in areceived baseband signal to α and β;

FIG. 47(b) is a diagram showing an eye opening of a received waveform inthe received baseband signal to α and β;

FIG. 48(a) is a diagram showing results of calculation of transmissionline PMD versus 20 GHz component intensity when transmission isperformed using a 40 Gb/s NRZ signal;

FIG. 48(b) is a diagram showing results of calculation of transmissionline PMD versus eye opening penalty when transmission is performed usinga 40 Gb/s NRZ signal;

FIG. 49(a) is a diagram showing results of calculation of transmissionline PMD versus 20 GHz component intensity when transmission isperformed using a 40 Gb/s OTDM signal;

FIG. 49(b) is a diagram showing results of calculation of transmissionline PMD versus eye opening penalty when transmission is performed usinga 40 Gb/s OTDM signal;

FIG. 50(a) is a diagram showing a relationship of transmission line PMDversus eye opening penalty when a delay quantity Δτ_(c) is the smallest;

FIG. 50(b) is a diagram showing a relationship of transmission line PMDversus eye opening penalty when a delay quantity Δτ_(c) is the largest;

FIGS. 51(a) and 51(b) are diagrams illustrating a case where a delayquantity Δτ exceeds one time slot;

FIGS. 52 and 53 are block diagrams showing a structure of an opticaltransmission system according to a third embodiment of this invention;

FIG. 54 is a block diagram showing a structure of an opticaltransmission system according to the third embodiment of this invention;

FIGS. 55 and 56 are block diagrams of an optical transmission systemaccording to a first modification of the third embodiment of thisinvention;

FIG. 57 is a block diagram of an optical transmission system accordingto a second modification of the third embodiment of this invention;

FIG. 58 is a block diagram according to the second modification of thethird embodiment of this invention;

FIG. 59 is a block diagram of an optical transmission system accordingto a third modification of the third embodiment of this invention;

FIG. 60 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a fourth modification of the third embodiment ofthis invention is applied;

FIG. 61 is another block diagram of an optical transmission systemaccording to a fifth modification of the third embodiment of thisinvention;

FIG. 62 is a block diagram of an optical transmission system accordingto a fourth embodiment of this invention;

FIG. 63 is a block diagram of an optical transmission system accordingto a first modification of the fourth embodiment of this invention;

FIG. 64 is a block diagram of an optical transmission system accordingto a second modification of the fourth embodiment of this invention;

FIG. 65 is a diagram showing a structure of another delay quantitycompensator;

FIG. 66 is a diagram showing a structure of an evaluation experimentalsystem for dispersion compensation tolerance after 1.3 μm SMF 50 kmtransmission in 40 Gb/s optical time division multiplexing;

FIG. 67 is a diagram showing results of an evaluation experiment in theexperimental system in FIG. 66;

FIG. 68 is a diagram showing a relationship (simulation results) between40 GHz component intensity and eye opening to a dispersion quantity of a40 Gb/s OTDM signal;

FIG. 69 is a diagram showing a structure of an experimental system atthe time of DSF 100 km transmission;

FIG. 70 is a diagram showing experiment results in the experimentalsystem in FIG. 69;

FIG. 71(a) is a diagram showing a relationship (simulation results)between 40 GHz component intensity and eye opening to a dispersionquantity of a 40 Gb/s NRZ signal (α=−0.7);

FIG. 71(b) is a diagram showing experimental results at the time of DSF100 km transmission when the temperature is changed from −35 to +65° C.;

FIG. 72(a) is a diagram showing results of simulation in the case of a40 Gb/s NRZ signal (α=+0.7);

FIG. 72(b) is a diagram showing results of simulation in the case of a40 Gb/s RZ signal (α=0, Duty=50%);

FIG. 73 is a diagram showing an experimental system for researchingtransmission waveform deterioration due to PMD in a 40 Gb/s signal;

FIG. 74 is a diagram showing results of a power penalty evaluationexperiment to an optical delay difference Δτ on a 40 Gb/s OTDM signaland an NRZ signal; and

FIG. 75 is a diagram showing experimental results showing Δτ dependencyof 20 GHz component intensity when γ=0.5 in a 40 Gb/s NRZ system.

BEST MODE FOR CARRYING OUT THE INVENTION

(A) Description of a Basic Structure of the Invention

(A1) Description of a Structure of a First Basic Block

FIG. 1 is a block diagram showing a structure of a first basic block ofa dispersion compensation controlling apparatus of this invention, whichcomprises, as shown in FIG. 1, a polarization-mode dispersioncompensator 7 a disposed in a transmission line 6 a and a dispersioncompensation controlling apparatus 251 a.

Here, the transmission line 6 a is an optical fiber transmission line.The polarization-mode dispersion compensator 7 a receives a controlsignal from the dispersion compensation controlling apparatus 251 a tocompensate polarization-mode dispersion generated in a transmittedoptical signal.

The dispersion compensation controlling apparatus 251 a monitors a stateof polarization-mode dispersion generated in an optical signaltransmitted over the transmission line 6 a on the basis of the receivedoptical signal, and controls the polarization-mode dispersioncompensator 251 a according to a result of the monitoring, whichcomprises a first specific frequency component detecting unit 2 a, afirst intensity detecting unit 3 a and a polarization-mode dispersioncontrolling unit 220 a.

A term “dispersion” is generally used to mean “chromatic dispersion”. Inthis structure, the term “dispersion” is used to mean “polarization-modedispersion”. Accordingly, the dispersion compensation controllingapparatus 251 a according to this structure represents“polarization-mode dispersion controlling apparatus”.

The first specific frequency component detecting unit 2 a detects afirst specific frequency component in a baseband spectrum in atransmission optical signal inputted to a receiving side over atransmission fiber as the transmission line 6 a. The first intensitydetecting unit 3 a detects information on an intensity of the firstspecific frequency component detected by the first specific frequencycomponent detecting unit 2 a. The polarization-mode dispersioncontrolling unit 220 a controls a polarization-mode dispersion quantityof the transmission line such that the intensity of the first specificfrequency component detected by the first specific frequency componentdetecting unit 2 a becomes the maximum.

When the above transmission optical signal is an RZ optical signal or anoptical time division multiplex signal, the first specific frequencycomponent detecting unit 2 a may detect a frequency corresponding to abit rate as the first specific frequency component. When the abovetransmission optical signal is in any optical modulation system, thefirst specific frequency component detecting unit 2 a may detect afrequency corresponding to ½ of a bit rate as the first specificfrequency component.

A dispersion compensation controlling method of this invention comprisesthe steps of a first specific frequency component detecting step ofdetecting a first specific frequency component in a baseband spectrum ina transmission optical signal inputted to a receiving side over atransmission fiber as a transmission line, a first intensity detectingstep of detecting information on an intensity of the above firstspecific frequency component detected at the first specific frequencycomponent detecting step, and a polarization-mode dispersion controllingstep of controlling a polarization-mode dispersion quantity of thetransmission line 6 a such that the intensity of the first specificfrequency component detected at the first intensity detecting stepbecomes the maximum.

(A2) Description of a Structure of a Second Basic Block

FIG. 2 is a block diagram showing a structure of a second basic block ofa dispersion compensation controlling apparatus of this invention, whichcomprises, as shown in FIG. 2, a chromatic dispersion compensator 206 aand a polarization-mode dispersion compensator 7 a disposed in atransmission line 6 a, and a dispersion compensation controllingapparatus 251 b. The transmission line 6 a is an optical fibertransmission line. The chromatic dispersion compensator 206 a receives acontrol signal from the dispersion compensation controlling apparatus251 b to compensate a chromatic dispersion quantity generated in atransmitted optical signal. The polarization-mode dispersion compensator7 a receives a control signal from the dispersion compensationcontrolling apparatus 251 b to compensate polarization-mode dispersiongenerated in a transmitted optical signal.

The dispersion compensation controlling apparatus 251 b monitors statesof chromatic dispersion and polarization-mode dispersion generated in anoptical signal transmitted over the transmission line 6 a on the basisof a received optical signal, and controls the chromatic dispersioncompensator 206 a and the polarization-mode dispersion compenstor 7 aaccording to results of the monitoring, which comprises a first specificfrequency component detecting unit 2 a, a first intensity detecting unit3 a, a polarization-mode dispersion controlling unit 220 a, a secondspecific frequency component detecting unit 222 a, a second intensitydetecting unit 223 a and a chromatic dispersion controlling unit 224 a.

A term “dispersion” is generally used to mean “chromatic dispersion”. Inthis structure, the term “dispersion” is used to mean both“polarization-mode dipsersion” and “chromaic dispersion”. Inconsequence, the dispersion compensation controlling apparatus 251 baccording to this structure represents “polarization-modedispersion-chromatic dispersion compensation controlling apparatus”.

The first specific frequency component detecting unit 2 a detects afirst specific frequency component in a baseband spectrum in atransmission optical signal inputted to a receiving side over atransmission line as the transmission line 6 a. The first intensitydetecting unit 3 a detects information on an intensity of the firstspecific frequency component detected by the first specific frequencycomponent detecting unit 2 a. The polarization-mode dispersioncontrolling unit 220 a controls a polarization-mode dispersion quantityof the transmission line 6 a such that the intensity of the firstspecific frequency component detected by the first intensity detectingunit 3 a becomes the maximum. The second specific frequency componentdetecting unit 222 a detects a second specific frequency component inthe baseband spectrum in the transmission optical signal. The secondintensity detecting unit 223 a detects information on an intensity ofthe above second specific frequency component detected by the secondspecific frequency component detecting unit 222 a. The chromaticdispersion controlling unit 224 a controls a chromatic dispersionquantity of the transmission line 6 a such that the intensity of thesecond specific frequency component detected by the second intensitydetecting unit 223 a becomes the maximum or the minimum.

When the above transmission optical signal is an NRZ optical signal, thefirst specific frequency component detecting unit 2 a may detect afrequency corresponding to ½ of a bit rate as the first specificfrequency component, while the second specific frequency componentdetecting unit 222 a may detect a frequency corresponding to the bitrate as the second specific frequency component.

A dispersion compensation controlling method of this invention comprisesthe steps of a first specific frequency component detecting step ofdetecting a first specific frequency component in a baseband spectrum ina transmission optical signal inputted to a receiving side over atransmission fiber as a transmission line, a first intensity detectingstep of detecting information on an intensity of the above firstspecific frequency component detected at the first specific frequencycomponent detecting step, a polarization-mode dispersion controllingstep of controlling a polarization-mode dispersion quantity of thetransmission line 6 a such that the intensity of the first specificfrequency component detected at the first intensity detecting stepbecomes the maximum, a second specific frequency component detectingstep of detecting a second specific frequency component in the basebandspectrum of the transmission optical signal, a second intensitydetecting step of detecting information on an intensity of the secondspecific frequency component detected at the second specific frequencycomponent detecting step, and a chromatic dispersion controlling step ofcontrolling a chromatic dispersion quantity of the transmission line 6 asuch that the intensity of the second specific frequency componentdetected at the second intensity detecting step becomes the maximum orthe minimum.

(A3) Description of a Structure of a Third Basic Block

FIG. 3 is a diagram showing a structure of a third basic block of adispersion compensation controlling apparatus of this invention, whichcomprises, as shown in FIG. 3, a chromatic dispersion compensator 206 aand a polarization-mode dispersion compensator 7 a disposed in atransmission line 6 a, and a dispersion compensation controllingapparatus 251 c. The transmission line 6 a is an optical fibertransmission line. The chromatic dispersion compensator 206 a receives acontrol signal from the dispersion compensation controlling apparatus251 c to compensate a chromatic dispersion quantity generated in atransmitted optical signal. The polarization-mode dispersion compensator7 a receives a control signal from the dispersion compensationcontrolling apparatus 251 c to compensate polarization-mode dispersiongenerated in a transmitted optical signal.

The dispersion compensation controlling apparatus 251 c monitors statesof chromatic dispersion and polarization-mode dispersion generated in anoptical signal transmitted over the transmission line 6 a on the basisof a received optical signal, and controls the chromatic dispersioncompensator 206 a and the polarization-mode dispersion compensator 7 aaccording to results of the monitoring, which comprises a first specificfrequency component detecting unit 2 a, a first intensity detecting unit3 a, a polarization-mode dispersion controlling unit 220 a and achromatic dispersion controlling unit 224 a.

A term “dispersion” is generally used to mean “chromatic dispersion”. Inthis structure, the term “dispersion” is used to mean both“polarization-mode dispersion” and “chromatic dispersion”. Inconsequence, the dispersion compensation controlling apparatus 251according to this structure represents “polarization-modedispersion-chromatic dispersion compensation controlling apparatus”.

The first specific frequency component detecting unit 2 a detects afirst specific frequency component in a baseband spectrum in atransmission optical signal inputted to a receiving side over atransmission fiber as the transmission line 6 a. The first intensitydetecting unit 3 a detects information on an intensity of the abovefirst specific frequency component detected by the first specificfrequency component detecting unit 2 a. The polarization-mode dispersioncontrolling unit 220 a controls a polarization-mode dispersion quantityof the transmission line 6 a such that the intensity of the firstspecific frequency component detected by the first intensity detectingunit 3 a becomes the maximum. The chromatic dispersion controlling unit224 a controls a chromatic dispersion quantity of the transmission line6 a such that the intensity of the first specific frequency componentdetected by the first intensity detecting unit 3 a becomes the maximumor the minimum.

When the above transmission optical signal is an RZ optical signal or anoptical time division multiplex signal, the first specific frequencycomponent detecting unit 2 a may detect a frequency corresponding to abit rate or ½ of the bit rate as the first specific frequency component.When the above transmission optical signal is an NRZ optical signal, thefirst specific frequency component detecting unit 2 a may detect afrequency corresponding to ½ of the bit rate as the first specificfrequency component.

The chromatic dispersion controlling unit 206 a may set a chromaticdispersion control quantity in the chromatic dispersion compensator 206a disposed in the transmission line 6 a such that the intensity of thefirst specific frequency component detected by the first intensitydetecting unit 3 a becomes the maximum or the minimum. The chromaticdispersion controlling unit 206 a may comprise a chromatic dispersionquantity detecting unit for detecting a chromatic dispersion quantity ofthe above transmission optical signal from the intensity of the abovefirst specific frequency component detected by the first intensitydetecting unit 3 a by performing an operation with a predeterminedsecond function, and a chromatic dispersion control quantity settingunit for setting a chromatic dispersion control quantity in thechromatic dispersion compensator 206 a on the basis of the abovechromatic dispersion quantity detected by the chromatic dispersionquantity detecting unit in order to compensate chromatic dispersion ofthe above transmission optical signal. The chromatic dispersioncontrolling unit 206 a may feedback-control the chromatic dispersioncompensator 206 a disposed in the transmission line 6 a such that theintensity of the first specific frequency component detected by thefirst intensity detecting unit 3 a beocmes the maximum or the minimum.

A dispersion compensation controlling method of this invention comprisesthe steps of a first specific frequency component detecting step ofdetecting a first specific frequency component in a baseband spectrum ina transmission optical signal inputted to a receiving side over atransmission fiber as a transmission line, a first intensity detectingstep of detecting information on an intensity of the above firstspecific frequency component detected at the first specific frequencycomponent detecting step, a polarization-mode dispersion controllingstep of controlling a polarization-mode dispersion quantity of thetransmission line 6 a such that the intensity of the first specificfrequency component detected at the first intensity detecting stepbecomes the maximum, and a chromatic dispersion controlling step ofcontrolling a chromatic dispersion quantity of the transmission line 6 asuch that the intensity of the first specific frequency componentdetected at the first intensity detecting step becomes the maximum orthe minimum.

(A4) Description of Polarization-Mode Dispersion

Hereinafter, description will be made of polarization-mode dispersionwith respect to the first to third basic blocks.

The polarization-mode dispersion controlling unit 220 a may set apolarization-mode dispersion control quantity in the polarization-modedispersion compensator 7 a disposed in the transmission line 6 a suchthat the intensity of the first specific frequency component detected bythe first intensity detecting unit 3 a becomes the maximum.

The polarization-mode dispersion controlling unit 220 a may comprise apolarization-mode dispersion quantity detecting unit for detecting apolarization-mode dispersion quantity of the above transmission opticalsignal from the intensity of the above first specific frequencycomponent detected by the first intensity detecting unit 3 a by using afirst function which is a function representing an intensity of afrequency component in a baseband spectrum in an optical waveformforming an arbitrary transmission optical signal and in which thefrequency information and parameters showing a polarization-modedispersion quantity are variables, and a parameter setting unit foroutputting a parameter setting control signal having parameterinformation as a control quantity for compensating polarization-modedispersion of the above transmission optical signal on the basis of theabove polarization-mode dispersion quantity detected by thepolarization-mode dispersion quantity detecting unit to thepolarization-mode dispersion compensator 7 a.

The dispersion compensation controlling apparatus 251 a (or 251 b or 251c) may further comprise a third specific frequency component detectingunit for detecting a third specific frequency component in the basebandspectrum of the transmission optical signal, a third intensity detectingunit for detecting information on an intensity of the above thirdspecific frequency component detected by the third specific frequencycomponent detecting unit. Besides, the polarization-mode dispersioncontrolling unit 220 a may comprise a polarization-mode dispersionquantity detecting unit for detecting a polarization-mode dispersionquantity of the above transmission optical signal from the intensity ofthe first specific frequency component and the intensity of the thirdspecific frequency component detected by the first intensity detectingunit and the third intensity detecting unit, respectively, by using afirst function which is a function showing an intensity of a frequencycomponent in a baseband spectrum in an optical waveform forming anarbitrary transmission optical signal and in which the frequencyinformation and parameters showing a polarization-mode dispersionquantity are variables, and a parameter setting unit for outputting aparameter setting control signal having parameter information as acontrol quantity for compensating polarization-mode dispersion of theabove transmission optical signal on the basis of the abovepolarization-mode dispersion quantity detected by the polarization-modedispersion quantity detecting unit to the polarization-mode dispersioncompensator.

The above parameter information may be at least either a delay quantityΔτ between two polarization modes or a splitting ratio γ of an opticalintensity to the above two polarization modes, and the parameter settingunit may output a parameter setting control signal for setting the aboveparameter information to the polarization-mode dispersion compensatordisposed in a receiving terminal apparatus which is a receiving terminalof the above transmission optical signal.

Further, the parameter setting unit may output a parameter settingcontrol signal for setting the above parameter information to apolarization-mode dispersion compensator disposed in a transmittingterminal apparatus transmitting the above transmission optical signal ora repeating apparatus amplifying and repeating the above transmissionoptical signal, or output a first parameter setting control signal forsetting a splitting ratio of an optical intensity to twopolarization-mode to a first polarization-mode dispersion compensatordisposed at an arbitrary position on the transmission line 6 a, whileoutputting a second parameter setting control signal for setting a delayquantity between the above two polarization modes to a secondpolarization-mode dispersion compensator arranged in a rear stage of thefirst polarization-mode dispersion compensator.

The dispersion compensation controlling apparatus may further comprise acompensation quantity optimization controlling unit for superimposing apredetermined low frequency signal set in advance on the parametersetting control signal outputted from the parameter setting unit, andcontrolling a parameter setting in the parameter setting unit such thatthe above low frequency signal component included in the intensity ofthe above first specific frequency signal from the first intensitydetecting unit 3 a becomes zero so as to optimize a compensationquantity of polarization-mode dispersion of the above transmissionoptical signal.

The compensation quantity optimization controlling unit may superimposetwo low frequency signals having low frequency components different fromeach other as the above predetermined low frequency signal on the aboveparameter setting control signal, control a setting of a splitting ratioof an optical intensity to two polarization modes in the parametersetting unit such that either one of the above two low frequency signalcomponents included in the intensity of the above first specificfrequency component from the first intensity detecting unit 3 a becomeszero, and control a setting of a delay quantity between the above twopolarization modes in the parameter setting unit such that the other ofthe two low frequency signal components included in the intensity of theabove first specific frequency component from the first intensitydetecting unit 3 a becomes zero. In addition, the compensation quantityoptimization controlling unit may switch a setting control on thesplitting ratio of an optical intensity to the above two polarizationmodes and a setting control on the delay quantity between twopolarization modes with respect to time, and perform the settingcontrols.

The distribution compensation controlling unit may still furthercomprise a sweep controlling unit for largely sweeping and controllingthe parameters showing the above polarization-mode dispersion quantityto be given by the polarization-mode dispersion compensator 7 a when asystem is actuated or the system is re-actuated.

The polarization-mode dispersion controlling unit 220 a mayfeedback-control at least either a polarization controller or aninter-polarization-mode delay unit disposed in the transmission line 6 asuch that the intensity of the first specific frequency componentdetected by the first intensity detecting unit 3 a becomes the maximum.The inter-polarization-mode delay unit may be configured as a devicesplitting polarization-mode components by a polarization beam splitter,giving a delay difference between the polarization-mode components by avariable optical delay path and multiplexing the polarization-modecomponents. The inter-polarization-mode delay unit may be configured asa device in which a plurality of polarization maintaining fibers havingdifferent polarization dispersion values are arranged in parallel andthe polarization maintaining fibers transmitting an optical signal areswitched by an optical switch according to a polarization-modedispersion quantity of the transmission line 6 a.

Further, the polarization-mode dispersion controlling unit 220 a mayperform a control in a first control mode in which any one of an azimuthangle of a ¼ wave plate, an azimuth angle of a ½ wave plate in thepolarization controller and a delay quantity between polarization modesof the inter-polarization-mode delay unit such that the intensity of thefirst specific frequency component becomes the maximum while theremaining control parameters among the above azimuth angles and thedelay quantity between polarization modes are fixed, after the firstcontrol mode, perform a control in a second control mode in which one ofthe remaining control parameters is changed such that the intensity ofthe first specific frequency component becomes the maximum while thecontrol parameter having been first changed and the other one of theremaining control parameters are fixed, finally perform a control in thethird control mode in which the other one of the remaining controlparameters is changed such that the intensity of the first specificfrequency component becomes the maximum while the control parameterhaving been first changed and the one of the control parameters arefixed.

In addition, the polarization-mode dispersion controlling unit 220 a mayperform a control in a fourth control mode in which any one of anazimuth angle of a ¼ wave plate, an azimuth angle of a ½ wave plate inthe polarization controller and a delay quantity between polarizationmodes of the inter-polarization-mode delay unit is changed such that theintensity of the first specific frequency component increases while theremaining parameters among the above azimuth angles and the delayquantity between polarization modes are fixed, after the fourth controlmode, perform a control in a fifth control mode in which one of theremaining control parameters is changed such that the intensity of thefirst specific frequency component increases while the control parameterhaving been first changed and the other one of the remaining controlparameters are fixed, finally perform a control in a sixth mode in whichthe other one of the remaining parameters is changed such that theintensity of the first frequency component increases while the controlparameter having been first changed and the one of the remaining controlparameters are fixed, after that, repeatedly execute the above fourthcontrol mode, the fifth control mode and the sixth control mode untilthe intensity of the first specific frequency component becomes themaximum.

Further, the dispersion compensation controlling apparatus may stillfurther comprise a compensation quantity optimization controlling unitfor superimposing a predetermined low frequency signal set in advance ona control signal to be outputted from the polarization-mode dispersioncontrolling unit 220 a to the above polarization controller and theinter-polarization-mode delay unit, and control the above polarizationcontroller and the inter-polarization-mode delay unit such that theabove low frequency signal component included in the intensity of theabove first specific frequency component from the first intensitydetecting unit 3 a becomes zero so as to optimize a compensationquantity of polarization-mode dispersion of the above transmissionoptical signal. The compensation quantity optimization controlling unitmay low-frequency-modulate an azimuth angle of a ¼ wave plate, anazimuth angle of a ½ wave plate in the polarization controller and adelay quantity between polarization modes of the inter-polarization-modedelay unit with different frequencies, detect the first frequencycomponent intensity in the baseband spectrum of a transmission opticalsignal, and optimize the azimuth angle of the ¼ wave plate and azimuthangle of the ½ wave plate in the above polarization controller and thedelay quantity between polarization modes of the inter-polarization-modedelay unit such that an intensity modulation component of a lowfrequency component included therein becomes zero. Further,polarization-mode dispersion controlling unit 220 a may control only thepolarization controller during system operation, and control theinter-polarization-mode delay unit at the time of start of systemoperation or when an element determining conditions of polarization-modedispersion in the transmission line 6 a is switched.

Further, the polarization-mode dispersion controlling unit 220 a mayfurther comprise a maximum allowable polarization-mode dispersionquantity setting means for setting a maximum allowable polarization-modedispersion quantity, set a delay quantity of the inter-polarization modedelay to a value above a lower limit value defined as a value obtainedby subtracting the maximum allowable polarization-mode dispersionquantity from one time slot and below an upper limit value defined as avalue having a magnitude twice the maximum allowable polarization-modedispersion quantity during system operation when feedback-controlling atleast either the polarization controller or the inter-polarization-modedelay unit disposed in the transmission line 6 a such that an intensityof a frequency component corresponding to ½ of a bit rate as the firstspecific frequency component detected by the first intensity detectingunit becomes the maximum. The polarization-mode dispersion controllingunit 220 a may set a delay quantity of the inter-polarization-mode delayunit at the time of system operation to the lower limit value, or set adelay quantity of the inter-polarization-mode delay unit at the time ofsystem operation to the upper limit value.

In addition, the inter-polarization-mode delay unit may be configuredwith a polarization maintaining fiber, or an inter-polarization-modevariable delay unit in a state where a delay quantity is fixed.

In summary, a polarization-mode dispersion quantity detecting method ofthis invention comprises the steps of a step of detecting a specificfrequency component in a baseband spectrum in a transmission opticalsignal inputted over a transmission fiber (specific frequency componentdetecting step), a step of next detecting an intensity of the abovespecific frequency component detected at the specific frequencycomponent detecting step (intensity detecting step), after that, a stepof detecting a polarization-mode dispersion quantity of the abovetransmission optical signal from information on the intensity of thespecific frequency component detected at the intensity detecting step byperforming a predetermined functional operation (dispersion quantitydetecting step).

At this time, at the dispersion quantity detecting step, the abovepredetermined functional operation is performed by using a functionwhich is a function representing an intensity of a frequency componentin a baseband spectrum in an optical waveform forming an arbitrarytransmission optical signal and in which the frequency information andparameters showing the polarization-mode dispersion quantity arevariables.

Further, the specific frequency at which a component is detected at thespecific frequency component detecting step may be set to a frequency atwhich a component of a baseband spectrum in the above transmissionoptical signal can be stably obtained with respect to time.

When the above transmission optical signal is an RZ optical signal or anoptical time division multiplex signal, the specific frequency at whichthe component is detected at the specific frequency component detectingstep may be set to a frequency corresponding to a bit rate. When theabove transmission optical signal is in any optical modulation system,the specific frequency at which the component is detected at thespecific frequency component detecting step may be set to a frequencycorresponding to ½ of a bit rate.

(A5) Description of a Chromatic Dispersion Compensating Method

Hereinafter, description will be made of a chromatic dispersioncompensating method with respect to the second and third basic blocks.

The chromatic dispersion controlling unit 224 a may set a chromaticdispersion control quantity in the chromatic dispersion compensator 206a disposed in the optical transmission line 6 a such that the intensityof the second specific frequency component detected by the secondintensity detecting unit 223 a becomes the maximum or the minimum.Further, the chromatic dispersion controlling unit 224 a may comprise achromatic dispersion quantity detecting unit for detecting a chromaticdispersion quantity of the above transmission optical signal from theintensity of the above second specific frequency component detected bythe second intensity detecting unit 223 a by performing an operationwith a predetermined second function, and a chromatic dispersion controlquantity setting unit for setting a chromatic dispersion controlquantity in the chromatic dispersion compensator 206 a on the basis ofthe above chromatic dispersion quantity detected by the chromaticdispersion quantity detecting unit in order to compensate chromaticdispersion of the above transmission optical signal.

The chromatic dispersion controlling unit 224 a may feedback-control achromatic dispersion controller 206 a disposed in the transmission line6 a such that the intensity of the second specific frequency componentdetected by the second intensity detecting unit 223 a becomes themaximum or the minimum.

Further, the second intensity detecting unit 223 a may outputinformation on the detected intensity of the above second specificfrequency component as a monitor signal.

In the dispersion compensation controlling method of this invention, theabove polarization-mode dispersion controlling step and the chromaticdispersion controlling step may be executed independently, or the abovepolarization-mode dispersion controlling step and the chromaticdispersion controlling step may be executed in time series.

(A6) Description of Other Supplementary Functions

The dispersion compensation controlling apparatus 251 a (or 251 b or 251c) of this invention may further comprise a timing extracting unit forextracting a timing of a received signal on the basis of the above firstspecific frequency component detected by the first specific frequencycomponent detecting unit 2 a. In the dispersion compensation controllingapparatus 251 a (or 251 b or 251 c), the first intensity detecting unit3 a may output information on the detected intensity of the above firstspecific frequency component as a monitor signal.

Incidentally, a term “dispersion” is used to mean both“polarization-mode dispersion” and “chromatic dispersion”. Thedispersion compensation controlling apparatus 251 a thus represents“polarization-mode dispersion controlling apparatus”. The dispersioncompensation controlling apparatus 251 b and the dispersion compensationcontrolling apparatus 251 c represent “polarization-modedispersion-chromatic dispersion compensation controlling apparatus”.

(B) Description of a First Embodiment of the Invention

FIG. 4 is a block diagram showing a structure of an optical transmissionsystem to which a dispersion compensation controlling apparatusaccording to a first embodiment of this invention is applied.

The optical transmission system 10 shown in FIG. 4 is an opticalcommunication system with a transmission rate B(b/s) (for example, 40Gb/s, 10 Gb/s or the like) adopting time division multiplexing (TDM:Time Division Multiplexing).

In the transmission system 10, an optical transmitter 2 as atransmitting terminal apparatus transmitting a transmission opticalsignal and an optical receiver 7 as a receiving terminal apparatusreceiving the transmission optical signal are connected over an opticaltransmission line (transmission fiber) 3, and a dispersion compensationcontrolling apparatus 1 is disposed on the receiving side.

The optical receiver 7 comprises a polarization-mode dispersioncompensator 4, an optical splitting unit 5 and an optical receiving unit6. The polarization-mode dispersion compensator 4 receives a controlsignal from the outside to compensate polarization-mode dispersiongenerated in a transmitted optical signal. The optical splitting unit 5is disposed in the optical receiver 7 to take out a part of thetransmission optical signal inputted to the receiving side over theoptical transmission line 3, and sends it out as monitor light to thedispersion compensation controlling apparatus 1. The optical receivingunit 6 receives the transmission optical signal.

The dispersion compensation controlling apparatus 1 monitors a state ofpolarization-mode dispersion generated in the optical signal transmittedover the optical transmission line 3 on the basis of the optical signaltaken out by the optical splitting unit 5, and controls thepolarization-mode dispersion compensator 4 according to a result of themonitoring. The dispersion compensation controlling apparatus 1comprises a photo receiver 11, a band-pass filter (fe BPF) 12, anintensity detector 13 and a polarization-mode dispersion controllingunit 90.

A term “dispersion” is generally used to mean “chormatic dispersion”. Inthis embodiment, the term “dispersion” is used to mean“polarization-mode dispersion”, thus the dispersion compensationcontrolling apparatus 1 represents “polarization-mode dispersioncompensation controlling apparatus 1”.

The photo receiver 11 receives the optical signal taken out by theoptical splitting unit 5, and converts it into an electric signal. Theband-pass filter 12 detects a first specific frequency component [fe(Hz) component] in a baseband spectrum of the transmission opticalsignal inputted to the receiving side over the optical transmission line3, which functions as a first specific frequency component detectingunit.

Here, the first specific frequency component is appropriately setaccording to a transmission rate or a signal waveform of an opticalsignal. For example, when the transmission optical signal is a 40 Gb/sRZ optical signal (or OTDM signal), the band-pass filter 12 detects afrequency (40 GHz) corresponding to the bit rate as the first specificfrequency component. When the transmission optical signal is a 10 Gb/sNRZ optical signal, the band-pass filter 12 detects a frequency (5 GHz)corresponding to ½ of the bit rate as the first specific frequency.

The intensity detector 13 detects an intensity of the above firstspecific frequency component detected by the band-pass filter 12, whichfunctions as a first intensity detecting unit. The intensity detector(first intensity detecting unit) 13 can output information on thedetected intensity of the above first specific frequency component as amonitor signal.

The polarization-mode dispersion controlling unit 90 detects apolarization-mode dispersion quantity of the above transmission opticalsignal from the intensity of the first specific frequency componentdetected by the intensity detector 13. This function is achieved by apolarization-mode dispersion quantity detecting unit 14 and a parametersetting circuit (parameter setting unit) 15.

The polarization-mode dispersion quantity detecting unit 14 detects apolarization-mode dispersion quantity of the above transmission opticalsignal from the intensity of the above first specific frequencycomponent detected by the intensity detecting unit 13 using a firstfunction which is a function showing an intensity of a frequencycomponent in a baseband spectrum in an optical waveform forming anarbitrary transmission optical signal, and in which the frequencyinformation and parameters showing a polarization-mode dispersionquantity are variables.

The parameter setting circuit 15 outputs a parameter setting controlsignal having parameter information as a control quantity forcompensating polarization-mode dispersion of the above transmissionoptical signal on the basis of the above polarization-mode dispersionquantity detected by the polarization-mode dispersion quantity detectingunit 14 to the polarization-mode dispersion compensator 4 disposed inthe optical transmission line 3. Incidentally, the parameter informationconcretely signifies a delay quantity (optical delay difference) Δτbetween two polarization modes.

In other words, in order to compensate polarization-mode dispersion of atransmission optical signal, the parameter setting circuit 15 outputs aparameter setting control signal for setting such parameter informationas to cancel a polarization-mode dispersion quantity detected by thepolarization-mode dispersion quantity detecting unit 14 to thepolarization-mode dispersion compensator 4 disposed in the opticalreceiver 7. The parameter setting circuit 15 sets the above parameterinformation such that the intensity of the above first specificfrequency component detected by the intensity detector 13 becomes themaximum, as will be described later.

Here, “such that an intensity of the first specific frequency componentbecomes the maximum” means that this control mode is a mode in which apolarization-mode dispersion quantity of the optical transmission line 3is controlled such that the intensity of the first specific frequencycomponent detected by the intensity detector 13 becomes maximum. Inconcrete, the polarization-mode dispersion quantity detecting unit 14extracts an intensity of a frequency component in a baseband spectrum inan optical waveform forming an arbitrary transmission optical signal,and detects a maximum point of the intensity of the first specificfrequency component using a function (first function) in which thefrequency information and parameters showing a polarization-modedispersion quantity are variables, a controlling method thereof will bedescribed in detail later.

Steps to execute a dispersion compensation control are as follows.Namely, the first specific frequency component in a baseband spectrum ina transmission optical signal inputted to the receiving side over thetransmission fiber as a transmission line is detected (first specificfrequency component detecting step), information on an intensity of theabove first specific frequency component detected at the first specificfrequency component detecting step is detected (first intensitydetecting step), and a polarization-mode dispersion quantity of theoptical transmission line 3 is controlled such that the intensity of thefirst specific frequency component detected at the first intensitybecomes the maximum (polarization-mode dispersion controlling step).

On the other hand, the polarization-mode dispersion compensator 4 in theoptical receiver 7 receives a parameter setting control signal from theparameter setting circuit 15 of the dispersion compensation controllingapparatus 1, and sets parameter information on the basis of the controlsignal, thereby compensating polarization-mode dispersion generated inan optical signal transmitted over the optical transmission line 3. Thepolarization-mode dispersion compensator 4 comprises, as shown in FIG.5, an optical axis adjuster 4D and a polarization maintaining fiber(PMF: Polarization Maintaining Fiber) 4A-4.

The optical axis adjuster (polarization controller) 4D adjusts an axiswhen a received light is inputted to the polarization maintaining fiber4A-4. Namely, the optical axis adjuster 4D adjusts a polarization stateof an inputted optical signal in a direction of the polarization primaryaxis of the polarization maintaining fiber 4A-4, also adjusts apolarization direction such that a code of a delay quantity to be givenby the polarization maintaining fiber 4A-4 cancels a delay quantity ofthe optical transmission line 3. The optical axis adjuster 4D comprises,for example, wave plates [a ½ wave plate (λ/2 plate) 4D-11 and a ¼ waveplate (λ/4 plate) 4D-12], and actuators 4D-13 and 4D-14 to perform apolarization control at a predetermined angle, adjusting to a major axisand a minor axis of the optical fiber being used.

The polarization maintaining fiber 4A-4 gives a predetermined delaydifference to two orthogonal polarization mode components, whichactually has a function as a delay quantity compensator of a fixed delayquantity. Namely, a function as a delay compensator (Δτ compensator)relates to a delay quantity Δτ between two poralization modes, which isachieved by the polarization maintaining fiber 4A-4.

An optical signal transmitted from the optical transmitter 2 passesthrough the optical transmission line 3, and is inputted to the opticalreceiver 7. In the optical axis adjuster 4D, a polarization state of theinputted optical signal is such that a polarization direction thereof isso adjusted that a code of a delay quantity given by the polarizationmaintaining fiber 4A-4 cancels a delay quantity in the opticaltransmission line 3. Further, a part of the transmission optical signalis sent out as monitor light from the optical splitting unit 5 to thedispersion compensation controlling apparatus 1, while the other part ofthe transmission optical signal is sent out to the optical receivingunit 6. The optical signal inputted to the dispersion compensationcontrolling apparatus 1 is converted from an optical signal to anelectric signal (O/E-converted) in the photo receiver 11, and the firstspecific frequency component (fe [Hz] component) in a baseband spectrumof the inputted transmission optical signal is detected by the band-passfilter 12. Further, information on an intensity of the first specificfrequency component is obtained by the intensity detector 13, and apolarization-mode dispersion quantity is detected by thepolarization-mode dispersion quantity detecting unit 14. A parametersetting control signal for setting parameter informationΔτ compensatinga polarization-mode dispersion of the transmission optical signal isoutputted from the parameter setting circuit 15 to the polarization-modedispersion compensator 4 disposed in the optical receiver 7, wherebypolarization-mode dispersion of the transmission optical signal iscompensated. An optimum control as the delay quantity compensator(optical axis adjuster 4D, polarization maintaining fiber 4A-4) isperformed in a polarization direction given by the optical axis adjuster4D.

Next, FIG. 6 and FIG. 7 show a structure of an experimental systemresearching an effect of polarization-mode dispersion on an opticalsignal in a 40 Gb/s optical transmission system. Results of the researchusing this experimental system are shown in FIGS. 8(a) through 8(e), 9,10(a) and 10(b).

The 40 G b/s optical transmission system 100 shown in FIG. 6 simulates a40 Gb/s optical communication system adopting optical time divisionmultiplexing. In the optical transmission system 100, an opticaltransmitter 101 and an optical receiver 102 are connected over anoptical transmission line 103. In order to simulatively givepolarization-mode dispersion to an optical signal, a polarizationcontroller (PC) 104 and a commercially available polarization-modedispersion emulator (PMD emulator) 105 are disposed in the opticaltransmission line 103. When an RZ optical signal is used, it is as wellpossible to monitor in this experimental system.

The optical transmitter 101 comprises a laser diode (LD) 101A, anoptical modulator 101B and an optical post-amplifier 101C. The laserdiode 101A is a signal light source. The optical post-amplifier 101C isan optical amplifier. The optical modulator 101B modulates light fromthe laser diode 101A into a 40 Gb/s optical time division multiplex(OTDM) optical signal. As the optical modulator 101B, here is used anoptical time division multiplexing modulator (hereinafter referred to asan OTDM modulator, occasionally) as shown in FIG. 15.

As shown in FIG. 15, the OTDM modulator 200 comprises a 20 GHz opticalswitching unit (1×2 switch) 201, a 20 Gb/s data modulating unit 202, aphase controlling unit (Phase controller) 203 and an opticalmultiplexing unit (Multiplexer) 204. The 20 GHz optical switching unit201 is a 1×2 optical switch. The 20 Gb/s data modulating unit 202performs data-modulation separately on optical clock signals in twosystems split by the 20 GHz optical switching unit 201, which comprisestwo modulating units (Two modulators). The phase controlling unit 203controls a phase difference between optical waves of the optical signalsin two systems outputted from the 20 Gb/s data modulating unit 202. Theoptical multiplexing unit 204 multiplexes the optical signals in twosystems outputted from the phase controlling unit 203. FIGS. 16(a)through 16(c) show optical waveforms (optical waveforms at positionsdenoted by {circle around (1)} through {circle around (3)} in FIG. 15)outputted from the optical switching unit 201, the data modulating unit202 and the optical multiplexing unit 204. FIGS. 16(a) through 16(c) arediagrams for illustrating an operating principle of the OTDM modulatoroperates. An optical waveform of the optical signal outputted from theOTDM modulator 200 shown in FIG. 15 corresponds to FIG. 16(c).

FIGS. 8(a) through 8(e) show deteriorated 40 Gb/s optical time divisionmultiplex waveforms when an optical delay difference Δτ is changed andgiven thereto by the PMD emulator 105.

Back to FIG. 6, in the PMD emulator 105, a polarization beam splitter(PBS) 105A is disposed at a branching portion of an optical waveguide ofa branching type, a polarization beam splitter (PBS9 105D is disposed ata combining portion of the optical waveguide 105E, an optical delay(optical delay) 105B is disposed in one parallel optical waveguide 105Fof the optical waveguide 105E, and an optical attenuator (opticalattenuator) 105C is disposed in the other one of the parallel opticalwaveguide 105G of the optical waveguide 105E, as shown in detail in FIG.7.

In the PMD emulator 105, an inputted optical signal is split into twopolarization components by the polarization beam splitter 105A. Apolarization component propagating through the parallel opticalwaveguide 105F is given a delay quantity (optical delay difference) Δτbetween two polarization modes by the optical delay 105B. In order toequalize optical losses of the parallel optical waveguides 105F and105G, a level of a polarization component propagating through theparallel optical waveguide 105G is adjusted by the optical attenuator105C. Further, the polarization components otuputted from the paralleloptical waveguides 105G and 105G are coupled still in an orthogonalstate by the polarization beam splitter 105D, and outputted.

The polarization controller 104 is disposed on the inputting side of thePMD emulator 105 to change a splitting ratio (hereinafter referred to asan optical intensity splitting ratio, occasionally) of an opticalintensity of polarization components at the polarization beam splitter(PBS: Polarization Beam Splitter) 105A of the PMD emulator 105.

As above, polarization-mode dispersion (delay quantity Δτ, opticalintensity splitting ratio γ) is simulatively given to an optical signalby the polarization controller 104 and the PMD emulator 105.

Again back to FIG. 6, the optical receiver 102 comprises an opticalpreamplifier 102A, an optical DEMUX (Demultiplex) 102B, a photodiode(PD) 102C, an amplifier 102D, an HBT D-FF 102E, a receiving unit 102F, aphotodiode (PD) 102G, a band-pass filter (BPF) 102H, a timing extractingunit (PLL) 102I, and a polarization-mode dispersion monitor (PMDmonitor) 102J.

In the optical receiver 102, a method of monitoring a state ofpolarization-mode dispersion uses an optical signal (the one inputted tothe photodiode 102G) split from the main signal system outputted fromthe optical preamplifier 102A shown in FIG. 6. Namely, the 40 Gb/soptical signal is converted into an electric signal (O/E-converted) bythe photodiode 102G, a 40 GHz component in a baseband spectrum of theoptical signal is extracted by the band-pass filter 102H of 40 GHz, andan intensity of the extracted 40 GHz component is measured by a powermeter of the PMD monitor 102J.

Next, Δτ (delay quantity) and γ dependency of receiver sensitivitydeterioration will be described with reference to FIG. 9 (denoted by I)and FIG. 10(a). Δτ and γ dependency of a 40 GHz component intensity willbe described with reference to FIG. 9 (denoted by II) and FIG. 10(b).Here, I and II in FIG. 9 show Δτ dependency of receiver sensitivitydeterioration and y dependency of a 40 GHz component intensity when anoptical intensity splitting ratio γ=0.5.

The one denoted by a reference character I in FIG. 9 depicts Δτdependency of receiver sensitivity deterioration (power penalty) due totransmission, while the other one denoted by a reference character II inFIG. 9 depicts Δτ dependency of a 40 GHz component intensity when anoptical intensity splitting ratio γ=0.5. As shown in the one denoted bya reference character II in FIG. 9, a 40 GHz component intensity is themaximum when Δτ=0 (ps), decreases with increasing Δτ, and is the minimumwhen Δτ=12.5 (ps). When Δτ further increases, the 40 GHz componentintensity turns to an increase, and is equal to the original intensitywhen it becomes an equal value of one time slot (Δτ=25 ps).

FIG. 10(b) depicts optical intensity splitting ratio γ dependency of the40 GHz component intensity when a delay quantity Δτ=10 (ps). As shown inFIG. 10(b), the 40 GHz component intensity is the minimum when γ=0.5,and is the maximum when γ=0 or 1.

On the other hand, as seen from the other one denoted by a referencecharacter I in FIG. 9 and FIG. 10(a), it is known that, from results ofmeasuring Δτ dependency of receiver sensitivity deterioration due totransmission, the best state with respect to Δτ in which the receiversensitivity deterioration due to transmission is the minimum is whenΔτ=0 (ps), and the best state with respect to the optical intensitysplitting ratio γ in which the receiver sensitivity deterioration due totransmission is the minimum is when γ=0 or 1. This coincides with a casewhere the 40 GHz component intensity is the maximum, as stated above.When γ=0.5 at which waveform deterioration due to polarization-modedispersion is the maximum, an allowable value (PMD tolerance) ofpolarization-mode dispersion at which the receiver sensitivitydeterioration after transmission is below 1 dB is about 0 ps.

FIG. 11 shows a structure of an experimental system of a 10 Gb/s NRZtransmission system whose value of the transmission rate B is not 40GHz. The 10 Gb/s NRZ transmission system 110 shown in FIG. 11 simulatesa 10 Gb/s optical communication system transmitting an NRZ signal.Results of researching an effect on an optical signal by thepolarization-mode dispersion control are shown in FIGS. 12(a) through12(j), 13, 14(a) and 14(b).

In the NRZ transmission system 110 shown in FIG. 11, an opticaltransmitter 111 and an optical receiver 112 are connected over anoptical transmission line 113. In order to simulatively givepolarization-mode dispersion to an optical signal, a polarizationcontroller (PC) 114 and a commercially available polarization-modedispersion emulator (PMD emulator) are disposed in the opticaltransmission line 113. In the optical transmission line 113, a 1.3 μmband zero-dispersion fiber (SMF) 113A of 50 km long is interposedaccording to the experiment.

The polarization controller 114 and the PMD emulator 115 simulativelygive polarization-mode dispersion (delay quantity Δτ and opticalintensity splitting ratio γ) to an optical signal, which are similar tothe polarization controller 104 and the PMD emulator 105 (refer to FIG.6) described above. The optical transmitter 111 comprises a laser diode(LD) 111A, an optical modulator 111B and an optical post-amplifier 111C.

The optical modulator 111B of the optical transmitter 111 modulateslight from the laser diode 111A into a 10 Gb/s NRZ optical signal. Asthe optical modulator 111B, here is used a lithium niobate opticalmodulator (LiNbO₃ optical modulator; not shown) of a Mach-Zehnder type.Incidentally, a 10 Gb/s NRZ waveform is generated by driving the lithiumniobate optical modulator by a 10 Gb/s NRZ electric signal.

FIGS. 12(a) through 12(j) show deteriorated 10 Gb/s NRZ waveforms at areceiving terminal in the case where an optical delay difference Δτ isvaried by the PMD emulator 115 and given to the 10 Gb/s NRZ opticalsignal outputted from the optical modulator 111B shown in FIG. 11. FIGS.12(a) through 12(e) show 10 Gb/s NRZ waveforms in the case transmissionover the SMF 113A is not performed, while FIGS. 12(f) through 12(j) show10 Gb/s NRZ waveforms in the case transmission over the SMF 113A isperformed.

Back to FIG. 11, the optical receiver 112 comprises an opticalpreamplifier 112A, a photodiode (PD) 112B, a receiving unit 112C, aphotodiode (PD) 112D, a band-pass filter (BPF) 112E, and apolarization-mode dispersion monitor (PMD monitor) 112F. A flow of aprocess of monitoring polarization-mode dispersion in the opticalreceiver 112 is as follows. Namely, an optical signal split from themain signal system is converted into an electric signal (O/E-converted)by the photodiode 112D, a 5 GHz component in a baseband spectrum of theoptical signal is extracted by the band-pass filter 112E of 5 GHz, andan intensity of the extracted 5 GHz component is measured by a powermeter as the PMD monitor 102F. Incidentally, since the 10 Gb/s NRZsignal does not have a 10 GHz component intensity, a 5 GHz componentthat is a half thereof is extracted and an intensity of it is measured.

FIG. 13 shows Δτ (delay quantity, group delay) dependency of receiversensitivity deterioration (power penalty) due to transmission (refer toa reference character III in FIG. 13), and Δτdependency of 5 GHzcomponent intensity (refer to a reference character IV in FIG. 13) whenan optical intensity splitting ratio γ=0.5. As shown in the one denotedby a reference character IV in FIG. 13, the 5 GHz component intensity isthe maximum when Δτ=0 (ps), similarly to the case of the transmissionrate 40 Gb/s OTDM signal. However, this case differs in point that acycle for Δτ is twice one time slot from the case of the transmissionrate 40 Gb/s OTDM signal.

FIG. 14(a) shows results of measuring optical intensity splitting ratioγ dependency of receiver sensitivity deterioration. FIG. 14(b) shows γdependency of 5 GHz component intensity when a delay quantity Δτ=40 ps.As shown in FIG. 14(a), the receiver sensitivity deterioration is themaximum when γ=0.5, and is the minimum when γ=0 or 1. As shown in FIG.14(b), the 5 GHz component intensity is the minimum when γ=0.5, and isthe maximum when γ=0 or 1, similarly to the case of the transmissionrate 40 Gb/s OTDM signal.

As seen from FIGS. 13, 14(a) and 14(b), when Δτ=0 (ps) with respect toΔτ, and when γ=0 or 1 with respect to γ, the state is the bast in whichreceiver sensitivity deterioration due to transmission is the minimum.This coincides with the case where the 5 GHz component is the maximum,as stated above.

When γ=0.5 at which waveform deterioration due to polarization-modedispersion is the maximum [refer to FIG. 14(a)], an allowable value (PMDtolerance) of polarization-mode dispersion at which receiver sensitivitydeterioration after transmission is below 1 dB is about 30 ps as shownby a reference character III in FIG. 13 when no fiber transmission isperformed.

As above, the PMD tolerance is almost inversely proportional to atransmission rate (bit rate) of an optical signal.

Namely, the greater the transmission rate of an optical signal and thelarger the transmission distance of an optical signal, the more aneffect by polarization-mode dispersion cannot be ignored.

Meanwhile, a method of detecting an intensity of a predeterminedfrequency component is as follows. Namely, the polarization-modedispersion quantity detecting unit 14 (refer to FIG. 4) extracts anintensity of a frequency component in a baseband spectrum in an opticalwaveform forming an arbitrary transmission optical signal, and detects amaximum point of the intensity of the first specific frequency componentusing a function (first function) in which the frequency information andparameters showing a polarization-mode dispersion quantity arevariables. The first function mentioned here is a functionquantitatively representing dependency of a 20 GHz component intensityin the 40 Gb/s RZ waveform on Δτ, or dependency of a 5 GHZ componentintensity in the 10 Gb/s NRZ waveform on Δτ, which is determinedaccording to Δτ and γ. Hereinafter, description will be made of thefirst function, in which a controlling method using this function willbe referred to as a control mode 1 in order to discriminate it from acontrol method to be described later.

Assuming that a time change of an optical intensity is F(t) when nopolarization-mode dispersion (delay quantity Δτ, optical intensitysplitting ratio γ) is given, a time change of an optical intensity whenpolarization-mode dispersion is given is expressed by the followingformula (1):

γF(t)+(1−γ)F(t+Δτ)  (1)

An electric field intensity of an electric signal after received isproportional to the value. The intensity detector 13 (refer to FIG. 4)detects a value of the square of it as a change with time of theintensity. Therefore, a baseband spectrum P(f) of an optical signal isgiven by the Fourier transform as shown in formula (2): $\begin{matrix}\begin{matrix}{{P(f)} = \quad {{\int{{\left\{ {{\gamma \quad {F(t)}} + {\left( {1 - \gamma} \right){F\left( {t + {\Delta\tau}} \right)}}} \right\} \cdot {\exp \left( {{\omega}\quad t} \right)}}{t}}}}^{2}} \\{= \quad {{{\gamma {\int{{{F(t)} \cdot {\exp \left( {{\omega}\quad t} \right)}}{t}}}} + {\left( {1 - \gamma} \right){\int{{{F\left( {t + {\Delta\tau}} \right)} \cdot {\exp \left( {{\omega}\quad t} \right)}}{t}}}}}}^{2}} \\{= \quad {{{\gamma {\int{{{F(t)} \cdot {\exp \left( {{\omega}\quad t} \right)}}{t}}}} + {\left( {1 - \gamma} \right) \cdot}}}} \\{\quad {\exp \left( {- {\omega\Delta\tau}} \right){\int{{{F(t)} \cdot {\exp \left( {{\omega}\quad t} \right)}}{t}}}}}^{2} \\{= \quad {{K(f)} \cdot {{\int{{{F(t)} \cdot \left( {{\omega}\quad t} \right)}{t}}}}^{2}}}\end{matrix} & (2) \\{{where}\quad {the}\quad {factor}\quad {of}\quad {proportionality}\quad {K(f)}\quad {is}\quad {expressed}\quad {by}\quad a\quad {formula}\quad (3)\text{:}} & \quad \\\begin{matrix}{{K(f)} = \quad {{\gamma + {\left( {1 - \gamma} \right) \cdot {\exp \left( {- {\omega\Delta\tau}} \right)}}}}^{2}} \\{= \quad {{\gamma + {\left( {1 - \gamma} \right) \cdot \left\{ {{\cos ({\omega\Delta\tau})} - {i \cdot {\sin ({\omega\Delta\tau})}}} \right\}}}}^{2}} \\{= \quad {1 - {4{\gamma \left( {1 - \gamma} \right)}{\sin^{2}\left( {\pi \quad f\quad {\Delta\tau}} \right)}}}}\end{matrix} & (3) \\{{{where}\quad \omega} = {2\pi \quad f}} & \quad\end{matrix}$

As above, since parameters Δτ and γ in terms of a state ofpolarization-mode dispersion are included in only K(f), it is possibleto separate it from a baseband spectrum |∫F(t)·exp(iωt)dt|² of anoptical signal without polarization-mode dispersion.

Since K=K(f,Δτ,γ) from the formula (3) when the above first specificfrequency component fe (Hz) is extracted by the band-pass filter 12(refer to FIG. 4) and an intensity thereof is detected by the intensitydetetor 13, K(f,Δτ,γ)=K(fe|Δτ,γ), so that K is dependent on the delayquantity Δτ and the optical intensity splitting ratio γ, where K(fe|Δτ,γ) is a function having variables Δτ and γ when fe is given.Accordingly, by measuring optical intensities (factors ofproportionality thereof) K(f) at two kinds of frequencies fe (Hz) on thereceiving side, it is possible to uniquely determine Δτ and γ in thetransmission line.

Moreover, since the formula (2) is established with respect to a generalformula F(t) representing an optical waveform, the above result that astate of polarization-mode dispersion can be detected with K(fe) isestablished irrespective of a signal form (NRZ or RZ) and a change inwaveform such as chromatic dispersion, nonlinear effect or the like.Incidentally, in the results of the experiment with the 10 Gb/s NRZtransmission system, the 5 GHz component intensity is large at the timeof fiber (SMF) transmission. A reason of this is that|∫F(t)·exp(iωt)dt|² is large, which meets a result that it isproportional to K(fe) with respect to polarization-mode dispersion.

When a state of polarization-mode dispersion is the bast, that is, whenwaveform deterioration due to polarization-mode dispersion is theminimum, it coincides with when an intensity of the fe(Hz) component isthe maximum, as stated above. Therefore, it is possible to detect apolarization-mode dispersion quantity using the formulae (2) and (3)when the polarization-mode dispersion compensator 4 disposed in theoptical transmission line 3 controls a delay quantity Δτ and compensatespolarization-mode dispersion. Accordingly, the above parameterinforamtion is a delay quantity Δτ between two polarization modes.

Namely, the formulae (2) and (3) are so generalized as to quantitativelydetect a state of polarization-mode dispersion (function of a delayquantity Δτ and an optical intensity splitting ratio γ) from a frequencycomponent intensity extracted from a baseband spectrum of an opticalsignal irrespective of a change in waveform such as a signal form (NRZ,RZ or the like) and a change in waveform such as chromatic dispersion,nonlinear effect or the like.

In other words, the formulae (2) and (3) correspond to the firstfunction (function in which frequency information and parameters showinga polarization-mode dispersion quantity are variables) of an intensityof a frequency component in a baseband spectrum in an optical waveformforming an arbitrary transmission optical signal (for example, a 40 Gb/sOTDM signal or a 10 Gb/s NRZ signal) used when the polarization-modedispersion quantity detecting unit 14 detects a polarization-modedispersion quantity of the transmission optical signal.

A flow of a signal in the optical transmission system 10 shown in FIG. 4is as follows. An optical signal at a transmission rate B (b/s)transmitted from the optical transmitter 2 is transmitted to the opticalreceiver 7 over the optical transmission line 3, a part of the opticalsignal transmitted over the optical transmission line 3 is taken out bythe optical splitting unit 5, and the optical signal (monitor light)taken out is sent to the dispersion compensation controlling apparatus 1in order to compensate polarization-mode dispersion generated in thetransmitted optical signal in the receiving terminal. In the dispersioncompensation controlling apparatus 1, a state of polarization-modedispersion generated in the optical signal transmitted over the opticaltransmission line 3 is monitored on the basis of the optical signaltaken out by the optical splitting unit 5, and a control by thepolarization-mode dispersion compensator 4 is performed according to aresult of the monitoring.

This polarization-mode dispersion quantity detecting step (detectingstep in a control mode 1) is as follows. In the dispersion compensationcontrolling apparatus 1, the optical signal taken out by the opticalsplitting unit 5 is first received by the photo receiver 11, convertedinto an electric signal (O/E-converted), then inputted to the band-passfilter 12.

The first specific frequency component [fe (Hz) component] in a basebandspectrum in the transmission optical signal inputted to the receivingside over the transmission fiber is detected by the band-pass filter 12(specific frequency component detecting step), and an intensity of theabove specific frequency component detected at the specific frequencycomponent detecting step is detected by the intensity detecting unit 13(intensity detecting step). Further, in the polarization-mode dispersionquantity detecting unit 14, a predetermined functional operation[functional operation using the above formulae (1) and (2)] is performedfrom information on the intensity of the above specific frequencycomponent detected at the intensity detecting step, whereby apolarization-mode dispersion quantity of the above transmission opticalsignal is detected (dispersion quantity detecting step).

Here, when the above transmission optical signal is a 40 Gb/s RZ opticalsignal or a 40 Gb/s OTDM signal, for example, the specific frequencywhose component is detected at the specific frequency componentdetecting step is set to a frequency (40 GHz) corresponding to the bitrate. Further, when the above transmission optical signal is a 10 Gb/sNRZ optical signal, the specific frequency whose component is detectedat the specific frequency component detecting step is set to a frequency(5 GHz) corresponding to ½ of the bit rate. Namely, the specificfrequency whose component is detected at the specific frequencycomponent detecting step is set to a frequency whose component in abaseband spectrum in the above transmission signal can be stablyobtained over a period of time.

In the polarization-mode dispersion quantity detecting unit 14(corresponding to the dispersion quantity detecting step), the abovepredetermined functional operation (first functional operation) isperformed using the first function that is a function which shows anintensity of a frequency component in a baseband spectrum in an opticalwaveform configuring an arbitrary transmission optical signal, and inwhich the frequency information and parameters showing apolarization-mode dispersion quantity are variables.

In the parameter setting circuit 15, a parameter setting control signalfor setting such parameter information (delay quantity Δτ) as to cancela polarization-mode dispersion quantity detected by thepolarization-mode dispersion quantity detecting unit 14 is outputted tothe polarization-mode dispersion compensator 4 disposed in the opticalreceiver 7 in order to compensate polarization-mode dispersion of thetransmission optical signal.

Namely, in the dispersion compensation controlling apparatus 1, a stateof polarization-mode dispersion [this is expressed as a function of adelay quantity Δτ and γ [the above formulae (2) and (3)]] of the opticaltransmission line 3 is detected by the polarization-mode dispersionquantity detecting unit 14 from a value of an fe (Hz) componentintensity detected by the intensity detector 13, and information thereonis fed back to the polarization-mode dispersion compensator 4 throughthe parameter setting circuit 15 in order to control thepolarization-mode dispersion compensator 4.

The polarization-mode dispersion compensator 4 sets parameterinformation on the basis of the control signal when receiving theparameter setting control signal so as to compensate polarization-modedispersion generated in an optical signal transmitted over the opticaltransmission line 3.

According to the dispersion compensation controlling apparatus 1according to the first embodiment of this invention, in the control mode1 (method using the first function), an intensity of the first specificfrequency component in a baseband spectrum in a transmission opticalsignal is detected, and a predetermined first functional operation isperformed to detect a polarization-mode dispersion quantity of thetransmission optical signal from the intensity of the detected firstspecific frequency component, so that polarization-mode dispersiongenerated in the transmission optical signal is easily detected.

As in the above way, a polarization-mode dispersion quantity is detectedat all times, and parameter information for compensatingpolarization-mode dispersion generated in a transmission optical signalis set on the basis of the detected polarization-mode dispersionquantity, whereby deterioration of a transmission wavform of the opticalsignal by compensating the polarization-mode dispersion, whichcontributes to long-distance transmission of a high-speed opticalsignal.

Incidentally, in FIG. 4 described above, it is possible to extract atiming of a received signal on the basis of the above first specificfrequency component detected by the band-pass filter (first specificfrequency component detecting unit) 12. FIG. 17 is a block diagramshowing a structure of an optical transmission system having adispersion compensation controlling apparatus 1M provided with a timingextracting unit 84 according to the first embodiment of this invention.The timing extracting unit 84 extracts a timing of a received signal onthe basis of the first specific frequency component detected by theband-pass filter 12. As the timing extracting unit 84, a PLL(Phase-Locked Loop) or the like is used. Incidentally, like referencecharacters in FIG. 17 designate like or corresponding parts in FIG. 4,further descriptions of which are thus omitted.

Since a fe (Hz) component is a signal in synchronization with a receivedwaveform as above, it is possible to take out a clock signal by thetiming extracting unit 84, and use it for discrimination or the like inthe optical receiver 7.

(B1) Description of a First Modification of the First Embodiment

FIG. 18 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a first modification of the first embodiment isapplied. The optical transmission system 210A shown in FIG. 18 is aswell an optical communication system with a transmission rate B (b/s)(for example, 40 Gb/s, 10 Gb/s or the like) adopting timing divisionmultiplexing. In the optical transmission system 210A, an opticaltransmitter 2 and an optical receiver 207 a are connected over anoptical transmission line (transmission fiber) 3, and a dispersioncompensation controlling apparatus 1M is provided on the receiving side.Here, the optical transmitter 2 and the optical transmission path 3 aresimilar to those described above, further descriptions of which are thusomitted.

The optical transmission system 210A differs from the one according tothe first embodiment in that signals in two systems are outputted fromthe optical splitting unit 205 a. Namely, a frequency value to detectthe specific frequency component is of one kind according to the firstembodiment, whereas frequency values to detect the specific frequencycomponent are of two kinds according to this modification. Hereinafter,the former will be referred to as a detection form 1, whereas the latterwill be referred to as a detection form 2, for the sake of explanation.Summarizing the control modes, the first embodiment adopts the controlmode 1 using the detection form 1, whereas this modification adopts thecontrol mode 1 using the detection form 2. As to a relationship betweenthe first function and its parameters, F=K(f, Δτ, γ). For this, thereceiving side using the detection form 1 can detect only one kind offrequency f₁ and an optical intensity K₁ thereat, but cannot determinevalues of Δτ and γ if the receiving side does not know either one of thevalues Δτ and γ, thus the receiving side cannot determine a controlvalue. In consequence, it is necessary to use a control system beingcapable even if values of Δτ and γ cannot be uniquely determined, suchas a maximum value control system or the like.

On the other hand, the receiving side using the detection form 2 candetect two kinds of frequencies f₁ and f₂ and optical intensities K₁ andK₂ thereat, so that the receiving side can determine the both values Δτand γ, and thus a control value. Meanwhile, since it is practicallydifficult for the receiving side to directly adjust a value of γ, γ isused for monitoring rather than for control (refer to an output of theparameter setting circuit 15 in FIG. 18). Incidentally, the detectionform 2 means a form in which different frequencies in two systems areused to perform one polarization-mode dispersion compensation (used inthe same meaning when chromatic dispersion compensation to be describedlater is performed).

The optical receiver 207 a comprises a polarization-mode dispersioncompensator 4, an optical splitting unit 205 a and an optical receiver6. The polarization-mode dispersion compensator 4 and the opticalreceiving unit 6 are similar to those described above, furtherdescriptions of which are thus omitted. The optical splitting unit 205 atakes out a part of a transmission optical signal inputted to thereceiving side over the optical transmission line 3, and sends it out asmonitor light in two systems to the dispersion compensation controllingapparatus 1M.

The dispersion compensation controlling apparatus 1M monitors a state ofpolarization-mode dispersion generated in an optical signal transmittedover the optical transmission line 3 on the basis of the optical signaltaken out by the optical splitting unit 205 a, and controls thepolarization-mode dispersion compensator 4 according to a result of themonitoring, which comprises photo receivers 11 a and 11 b, band-passfilters (fe BPF) 12 a and 12 b, intensity detecting units 13 a and 13 b,and a polarization-mode dispersion controlling unit 90 a. The photoreceivers 11 a and 11 b, the band-pass filters 12 a and 12 b, theintensity detectors 13 a and 13 b are similar to the photo receiver 11,the band-pass filter 12 and the intensity detector 13 described above,respectively, further descriptions of which are thus omitted.

Although a term “dispersion” is generally used to mean “chromaticdispersion”, the term “dispersion” is used to mean “polarization-modedispersion” in this modification, the dispersion compensationcontrolling apparatus 1M thus represents “polarization-mode dispersioncontrolling apparatus 1M”.

The polarization-mode dispersion controlling unit 90 a performs acontrol using the detection form 2 using the control mode 1. Namely, thepolarization-mode dispersion controlling unit 90 a detects apolarization-mode dispersion quantity of the above transmission opticalsignal from an intensity of the first specific frequency componentdetected by the intensity detector 13 b and an intensity of a thirdspecific frequency component detected by the intensity detector 13 b.This function is achieved by a polarization-mode dispersion quantitydetecting unit 14 and a parameter setting circuit 15. Incidentally, thepolarization-mode dispersion quantity detecting unit 14 and theparameter setting circuit 15 are similar to those described above,further descriptions of which are thus omitted.

A controlling method by the polarization-mode dispersion controllingunit 90 a is as follows. Namely, with two kinds of frequency information(first specific frequency component information and the third specificfrequency component information) obtained by the two intensity detectors13 a and 13 b, parameters Δτ and γ are determined as in a way of solvingsimultaneous equations with two unknowns in terms of the first function.Δτ is controlled, while γ is used for monitoring. Here, the firstfunction is an established form relating to dependency of the 40 GHzcomponent intensity in a 40 Gb/s OTDM waveform on Δτ, or dependency ofthe 5 GHz component intensity in a 10 Gb/s NRZ waveform on Δτ. When γcan be fed back to the transmitting side as well, it is possible tocontrol a splitting ratio of an optical intensity (as to thisembodiment, description will be made in another modification).

Namely, the dispersion compensation controlling apparatus 1M comprises athird specific frequency component detecting unit (band-pass filter 12b) detecting the third specific frequency component in a basebandspectrum in a transmission optical signal, and a third intensitydetecting unit (polarization-mode dispersion quantity detecting unit 14)detecting information on an intensity of the above third specificfrequency component detected by the third specific frequency componentdetecting unit. Besides, the polarization-mode dispersion controllingunit 90 a comprises the polarization-mode dispersion quantity detectingunit 14 detecting a polarization-mode dispersion quantity from theintensity of the first specific frequency component and the intensity ofthe third specific frequency component detected by the first intensitydetecting unit and the third intensity detecting unit (polarization-modedispersion quantity detecting unit 14), respectively, using the firstfunction which is a function representing an intensity of a frequencycomponent in a baseband spectrum in an optical waveform configuring anarbitrary transmission optical signal and in which the frequencyinformation and parameters showing a polarization-mode dispersionquantity are variables, and the parameter setting circuit 15 outputtinga parameter setting control signal having parameter information as acontrol quantity for compensating polarization-mode dispersion of theabove transmission optical signal on the basis of the abovepolarization-mode dispersion quantity detected by the polarization-modedispersion quantity detecting unit 14 to the polarization-modedispersion compensator 4. Incidentally, the parameter information is adelay quantity (optical delay difference) Δτ between two polarizationmodes. The parameter setting circuit 15 outputs a parameter settingcontrol signal for setting the above parameter information to thepolarization-mode dispersion compensator 4 disposed in the receivingterminal apparatus (optical receiver 7 a) which is a receiving terminalof the above transmission optical signal.

In the above structure, received light is split into two by the opticalsplitting unit 205 a, O/E-converted by the photo receivers 11 a and 11b, then inputted to the band-pass filters 12 a and 12 b. In theband-pass filter 12 a, the first specific frequency component in abaseband spectrum in the transmission optical signal inputted to thereceiving side over the optical transmission fiber is detected, while inthe band-pass filter 12 b, the third specific frequency component in thebaseband spectrum of the transmission optical signal inputted to thereceiving side over the transmission optical fiber is detected (specificfrequency component detecting step). Further, intensities of the abovefirst specific frequency component and the third specific frequencycomponent detected by the intensity detectors 13 a and 13 b at thespecific frequency component detecting step are detected (intensitydetecting step) Still further, in the polarization-mode dispersionquantity detecting unit 14, a polarization-mode dispersion quantity ofthe above transmission optical signal is detected from information onthe intensities of the above two kinds of specific frequency componentsdetected at the intensity detecting step by performing a predeterminedfunctional operation [functional operation using the above formulae (2)and (3)] (dispersion quantity detecting step).

By using that a time at which waveform deterioration due topolarization-mode dispersion is the minimum and a time at which the fe(Hz) component intensity is the maximum coincide, and since apolarization-mode dispersion quantity is detected in the control mode 1and the detection form 2, as above, it is possible to control a delayquantity Δτ to compensate polarization-mode dispersion by thepolarization-mode dispersion compensator 4 disposed in the opticaltransmission path 3.

It is possible as well to quantitatively detect a state ofpolarization-mode dispersion (function of a delay quantity Δτ) from afrequency component intensity extracted from a baseband spectrum of anoptical signal irrespective of a signal form (NRZ, RZ or the like) and awaveform change such as chromatic dispersion, nonlinear effect or thelike.

(B2) Description of a Second Modification of the First Embodiment

Although the polarization-mode dispersion compensator 4 is disposed onthe side of the optical receiver 7 in FIGS. 4, 17 and the like above, itis alternatively possible to dispose the polarization-mode compensator 4on the side transmitting signal light.

FIG. 19 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a second modification of the first embodiment ofthis invention is applied. The optical transmission system 10A shown inFIG. 19 is as well an optical communication system with a transmissionrate B (b/s) (for example, 40 Gb/s, 10 Gb/s or the like) adopting timedivision multiplexing. The optical transmission system 10A differs fromthe optical transmission system 10 according to the first embodiment inthat a polarization-mode dispersion compensator 4 is disposed in anoptical transmitter 2A. Namely, the optical transmission system 10Acomprises an optical transmitter 2A, an optical transmission line 3, anoptical splitting unit 5, an optical receiver 7A and a dispersioncompensation controlling apparatus 1. The optical transmitter 2Acomprises a signal light source 5, an optical modulator 9 and apolarization-mode dispersion compensator 4.

The dispersion compensation controlling apparatus 1 sends back a resultobtained by detecting a polarization state of an optical signal on theside of the optical receiver 7A up to the optical transmitter 2A that isthe transmitting side. This sending-back method may be a method ofpreparing another line with a low speed or a method of multiplexinginformation on a transmission optical signal in the opposite direction.A term “dispersion” is generally used to mean “chromatic dispersion”. Inthis modification, the term “dispersion” is used to mean“polarization-mode dispersion”, the dispersion compensation controllingapparatus 1 thus represents “polarization-mode dispersion controllingapparatus”.

The polarization-mode dispersion compensator 4 disposed in the opticaltransmitter 2A can change the optical intensity splitting ratio γ oftransmission light and send the light. Although not shown, an opticalamplifier is disposed on the output's side of the polarization-modedispersion compensator 4, and this optical amplifier transmits to theoptical transmission line 3. Incidentally, the other parts having thesame reference characters have the same or similar functions, furtherdescriptions of which are thus omitted. Since a frequency provided forintensity detection is one system, here is employed the detection form1.

Namely, in the dispersion compensation controlling apparatus 1 accordingto the second modification of the first embodiment, a parameter settingcircuit 15 outputs a parameter setting control signal for setting theabove parameter information to the polarization-mode dispersioncompensator 4 disposed in the optical transmitter 2A (transmittingterminal apparatus) transmitting the above transmission optical signal.

With the above structure, the optical transmission system 10A operatesin the almost similar manner to the optical transmission system 10 towhich the dispersion compensation controlling apparatus 1 according tothe firs t embodiment is applied. Here, the dispersion compensationcontrolling apparatus 10A uses the detection form 1 in the control mode1.

According to the dispersion compensation controlling apparatus 1according to the second modification of the first embodiment, it ispossible to attain similar effects to the first embodiment describedabove. In addition, it is possible to control a polarization directionsuch that the optical intensity splitting ratio γ is in the best state(γ=0 or 1) according to a state of the optical transmission path 3 bycontrolling the polarization-mode dispersion compensator 4 disposed inthe optical transmitter 2A, so that polarization-mode dispersiongenerated in the transmission optical signal is more effectivelycompensated.

Incidentally, the polarization-mode dispersion compensator 4 may beformed as a linear repeater or the like in the optical transmission line3.

FIG. 20 is a block diagram showing a structure of another opticaltransmission system to which the dispersion compensation controllingapparatus according to the second modification of the first embodimentis applied. The optical transmission system 210B shown in FIG. 20 is aswell an optical communication system with a transmission rate B (b/s)(for example, 40 Gb/s, 10 Gb/s or the like) adopting time divisionmultiplexing. The optical transmission system 210B differs from theoptical transmission system 10 according to the first embodiment in thata polarization-mode dispersion compensator 4 is disposed in an opticalrepeating apparatus (Optical Repeater) 214.

Namely, the optical transmission system 210 comprises the opticalrepeating apparatus 214 along with an optical transmitter 2, an opticaltransmission lines 3 and 3′ and an optical receiver 7A. The opticalrepeating apparatus 214 amplifies and repeats the above transmissionoptical signal, which comprises an optical repeater 7′ and a dispersioncompensation controlling apparatus 1.

The optical repeater 7′ receives signal light from the opticaltransmitter 2, and optically amplifies and transmits it to the opticalreceiver 7A, which comprises an optical repeating unit 6′ performingoptical amplification and optical re-transmission along with thepolarization-mode dispersion compensator 4 and an optical splitting unit5. Incidentally, the optical transmitter 2, the optical transmissionlines 3 and 3′, the optical receiver 7A and the dispersion compensationcontrolling apparatus 1 other than the above have similar functions tothose of the optical transmission system 10 according to the firstembodiment, further descriptions of which are thus omitted.

The dispersion compensation controlling apparatus 1 sends back a resultof detection on a polarization state of an optical signal by the opticalrepeater 7′ using the detection form 1 and the control mode 1 to theoptical transmitter 2 that is the transmitting side, and outputs aparameter setting control signal for setting the above parameterinformation to the polarization-mode dispersion compensator 4. Asending-back method may be a method of preparing another line with a lowspeed or a method of multiplexing information on a transmission opticalsignal in the opposite direction. Incidentally, the other parts denotedby the same reference characters have the same or similar functions,further descriptions of which are thus omitted.

With the above structure, the optical transmission system 210B operatesin the almost similar manner to the optical transmission system 10 towhich the dispersion compensation controlling apparatus 1 according tothe first embodiment using the detection form 1 and the control mode 1is applied. As this, it is possible to attain the same effects as thefirst embodiment described above. In addition, by controlling thepolarization-mode dispersion compensator 4 disposed in the repeatingapparatus 214, it is possible to more effectively compensatepolarization-mode dispersion generated in a transmission optical signalaccording to a state of the optical transmission line 3.

Further, a structure of the polarization-mode dispersion compensationmay be varied.

FIG. 21 is a block diagram showing an optical transmission system towhich still another dispersion compensation controlling apparatusaccording to the second modification of the first embodiment of thisinvention is applied. The optical transmission system 10B shown in FIG.21 is as well an optical communication system with a transmission rate B(b/s) (for example, 40 Gb/s, 10 Gb/s or the like) adopting time divisionmultiplexing.

The optical transmission system 10B differs from the opticaltransmission system 10 according to the first embodiment in that thepolarization-mode dispersion compensator 4 is divided into a γcompensator 4B′ and a Δτ compensator 4A, and disposed in an opticaltransmitter 2B and an optical receiver 7B, the other parts are similarto those of the optical transmission system 10 according to the firstembodiment. Namely, the optical transmission system 10B comprises adispersion compensation controlling apparatus 1A along with the opticaltransmitter 2B, an optical transmission line 3, and the optical receiver7B.

Here, the optical transmitter 2B is a transmitting terminal apparatustransmitting a transmission optical signal, which comprises a γcompensator 4B′ along with a signal light source 8 and the opticalmodulator 9. The optical transmission path 3 is a transmission fiber.The optical receiver 7B is a receiving terminal apparatus receiving atransmission optical signal, which has a Δτ compensator 4A along with anthe optical splitter 5 and an optical receiving unit 6.

The dispersion compensation controlling apparatus 1A is a controlapparatus for compensating polarization-mode dispersion generated in anoptical signal transmitted, using the control mode 1, which comprises aphoto receiver 11, a band-pass filer (fe BPF) 12, an intensity detector13, a polarization-mode dispersion quantity detecting unit 14, and aparameter setting circuit 15. The parameter setting circuit 15 comprisesa Δτ setting circuit 15A for setting Δτ, and a γ setting circuit 15B forsetting γ. Incidentally, since a frequency provided for intensitydetection is one system, here is employed the detection form 1.

Information on a polarization state detected by the polarization-modedispersion quantity detecting unit 14 is set in the γ compensator 4B′ inthe optical transmitter 2B by the γ setting circuit 15B in the parametersetting circuit 15, and set in the Δτ compensator 4A in the opticalreceiver 7B by the Δτ setting circuit 15A in the parameter settingcircuit 15 as well. The polarization-mode dispersion quantity detectingunit 14 and the parameter setting circuit 15 function as apolarization-mode dispersion controlling unit 90 b.

Namely, in the optical transmission system 10B, the dispersioncompensation controlling apparatus 1A sends information relating to γfrom which a polarization state of an optical signal is obtained on thereceiving side (the side of the optical receiver 7B) of an opticalsignal to the transmitting side (the side of the optical transmitter2B), so as to variably control the optical intensity splitting ratio γ.

In the dispersion compensation controlling apparatus 1A, the parametersetting circuit 15 outputs a first parameter setting control signal forsetting a splitting ratio γ of an optical intensity to two polarizationmodes to a first polarization-mode dispersion compensator (γ compensator4B′) disposed at an arbitrary position (in the optical transmitter 2B)in the transmission line, while outputting a second parameter settingcontrol signal for setting a delay quantity Δτ between the above twomodes to a second polarization-mode dispersion compensator (Δτcompensator 4A) arranged in the rear stage (in the optical receiver 7B)of the first polarization-mode dispersion compensator.

With the above structure, the optical transmission system 10B operatesin the almost similar manner to the optical transmission system 10 towhich the dispersion compensation controlling apparatus 1 according tothe first embodiment using the detection form 1 and the control mode 1is applied.

According to the dispersion compensation controlling apparatus 1A, it ispossible to attain the same effects as the first embodiment describedabove. In addition, it is possible to appropriately control both a delayquantity a Δτ and an optical intensity splitting ratio γ since the γcompensator 4B′ and the Δτ compensator 4A disposed in the opticaltransmitter 2B and the optical receiver 7B, respectively, areindependently controlled.

(B3) Description of a Third Modification of the First Embodiment

FIG. 22 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a third modification of the first embodiment ofthis invention is applied. The optical transmission system 10C shown inFIG. 22 is as well an optical communication system with a transmissionrate B (b/s) (for example, 40 Gb/s, 10 Gb/s or the like) adopting timedivision multiplexing. The optical transmission system 10C differs fromthe optical transmission system 10B according to the second modificationof the first embodiment in that a control on a delay quantity Δτ isperformed on an electric stage on the receiving side, the other parts ofwhich are almost similar to the optical transmission system 10B. In theoptical transmission system 10C, detection using the detection form 1and the control mode 1 is performed.

The optical transmission system 10C comprises an optical transmitter 2,an optical transmission line 3, an optical receiver 7C and a dispersioncompensation controlling apparatus 1B. The dispersion compensationcontrolling apparatus 1B comprises a band-pass filter (fe BPF) 12, anintensity detector 13, a polarization-mode dispersion quantity detectingunit 14 and a parameter setting circuit 215.

A term “dispersion” is generally used to mean “chromatic dispersion”. Inthis modification, the term “dispersion” is used to mean“polarization-mode dispersion”, the dispersion compensation controllingapparatus 1B thus represents “polarization-mode dispersion controllingapparatus 1B”.

The polarization-mode dispersion quantity detecting unit 14 and theparameter setting circuit 215 function as a polarization-mode dispersioncontrolling unit 90 c. The parameter setting circuit 215 comprises anoptical axis setting circuit 215A for setting a set value of an opticalaxis adjuster (polarization controlling unit) 4D, and a Δτ settingcircuit 15A.

A flow of a received optical signal in the optical transmission system10C is as follows. First, an optical axis of the received light isadjusted in the optical axis adjuster 4D in the optical receiver 7C,polarization-mode components are split by a polarization beam splitter(PBS) 17, and the both mode components are received and converted intoelectric signals (O/E-convertded) by photo receivers 11A and 11B. Adelay difference Δτ is given between both optical paths by a variabledelay element 18, after that, the signals are multiplexed by amultiplexing circuit 19, and undergo a light receiving process in anoptical receiving unit 6. Incidentally, the variable delay element willbe described later.

A part of the electric signal multiplexed by the multiplexing circuit 19is split and inputted to the dispersion compensation controllingapparatus 1B, an fe (Hz) component intensity is detected by theband-pass filter 12 and the intensity detector 13, a state ofpolarization-mode dispersion of the optical transmission line 3 isdetected by the polarization-mode dispersion quantity detecting unit 14,and the variable delay element 18 and the optical axis adjuster 4D aresuch controlled that the fe (Hz) component intensity becomes themaximum, inorder that the parameter setting circuit 215 compensatespolarization-mode dispersion.

In the above manner, it is possible to appropriately control a delayquantity Δτ like the dispersion compensation controlling apparatus 1Aaccording to the second modification of the first embodiment.

(B4) Description of a Fourth Modification of the First Embodiment

As an optical transmission system performing a control on a delayquantity Δτ on the electric stage on the receiving side, one shown inFIG. 23 is also possible. A controlling method in this case uses thecontrol mode 1 as well, but the method is slightly different. Since afrequency provided for intensity detection is one system, it means thathere is employed the detection form 1.

FIG. 23 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a fourth modification of the first embodiment ofthis invention is applied. The optical transmission system 10D is aswell an optical communication system with a transmission rate B (b/s)(for example, 40 Gb/s, 10 Gb/s or the like) adopting time divisionmultiplexing, which comprises an optical transmitter 2, an opticaltransmission line 3, an optical receiver 7D and a dispersioncompensation controlling apparatus 1B.

A term “dispersion” is generally used to mean “chromatic dispersion”. Inthis modification, the term “dispersion” is used to mean“polarization-mode dispersion”, the dispersion compensation controllingapparatus 1B thus represents “polarization-mode dispersion compensationcontrolling apparatus 1B”.

The optical receiver 7D splits inputted transmission signal light intothree directions, and controls them in only an electric stage. Theoptical receiver 7D comprises an X₁ polarizer 20A, an X₂ polarizer 20B,an X₃ polarizer 20C, photo receivers 11C, 11D and 11E connected thereto,respectively, an intensity variable element 21A connected to the photoreceiver 11C, a variable delay element 18A connected to the photoreceiver 11D, a variable delay element 18B connected to the photoreceiver 11E, intensity variable elements 21B and 21C connected to thevariable delay elements 18A and 18B, respectively, a multiplexingcircuit 19 and an optical receiving unit 6.

Here, the X₁ polarizer 20A, the X₂ polarizer 20B and the X₃ polarizer20C extract three components, that is, X₁, X₂ and X₃, respectively, ofStokes vector (Stokes vector) showing a polarization state of an opticalsignal. The photo receivers 11C, 11D and 11E O/E-convert the componentsof the optical signal, respectively.

The variable delay element 18A gives a delay quantity Δτ₂ correspondingto the Stokes vector X₂. The variable delay element 18B gives a delayquantity Δτ₃ corresponding to the Stokes vector X₃. Further, theintensity variable elements 21A, 21B and 21C give intensity ratios P1,P2 and P3 (here a relationship of P1+P2+P3=1 is satisfied),respectively. The intensity variable element 21A gives an intensityratio p1 corresponding to the Stokes vector X₁, the intensity variableelement 21B an intensity ratio P2 corresponding to the Stokes vector X₂,and the intensity variable element 21C an intensity ratio P3corresponding to the Stokes vector X₃. These five kinds of parameters(Δτ₂, Δτ₃, P1, P2 and P3) are appropriately controlled in order tomaximize the fe (Hz) component intensity. These intensity ratios P1, P2and P3 are parameters corresponding to a λ/4 plate azimuth (rotation)angle α and a λ/2 plate azimuth (rotation) angle β. The multiplexingcircuit 19 multiplexes output signals from the intensity variableelements 21A, 21B and 21C. The optical receiving unit 6 performs a lightreceiving process.

The dispersion compensation controlling apparatus 1B performs a Δτcontrol in the electric stage, which comprises a band-pass filter 12, anintensity detector 13, a polarization-mode dispersion quantity detectingunit 14 and a parameter setting circuit 215. The polarization-modedispersion quantity detecting unit 14 and the parameter setting circuit215 function as a polarization-mode dispersion controlling unit 90 c.The Δτ setting circuit 15A in the parameter setting circuit 215 inputscontrol signals to the variable delay element 18A and the variable delayelement 18B in the optical receiver 7D. The intensity setting circuit215B in the parameter setting circuit 215 inputs intensity ratios to theintensity variable elements 21A, 21B and 21C in the optical receiver 7D.

As an example of algorithm of a control on P1, P2 and P3, here isemployed a method of moving two among the three at a time. Namely, P1and P2 are varied while P3 is fixed such that P1+P2 is constant, therebycontrolling the fe (Hz) component intensity to be of the maximum value.Next, P2 and P3 are varied while P1 is fixed such that P2+P3 isconstant, thereby controlling the fe (Hz) component intensity to be ofthe maximum value. Further, P1 and P3 are varied while P2 is fixed suchthat P1+P3 is constant, thereby controlling the fe (Hz) componentintensity to be of the maximum value. Incidentally, it is needless tosay that the controlling method is possible in another manner.

A flow of a received optical signal in the optical transmission system10D is as follows. Transmission signal light inputted over the opticaltransmission line 3 is split into three in the optical receiver 7D,received by the photo receivers 11C, 11D and 11E through the polarizers(X₁ polarizer 20A, X₂ polarizer 20B and X₃ polarizer 20C) eachtransmitting only a corresponding polarization component, and convertedinto electric signals (O/E-converted). The optical components receivedby the photo receivers 11D and 11E are given delay quantities Δτ₂ andΔτ₃ by the variable delay elements 18A and 18B, respectively. Further,the three optical components, that is, outputs of these two systems andan output of the photo receiver 11C, undergo intensity ratio adjustmentby the intensity variable elements 21 a, 21B and 21C, respectively.

In this occasion, a part of the electric signal multiplexed by themultiplexing circuit 19 is split and inputted to the dispersioncompensation controlling apparatus 1B, the fe (Hz) component intensityis detected by the band-pass filter 12 and the intensity detector 13 inthe similar manner to the first embodiment, and a state ofpolarization-mode dispersion of the optical transmission line 3 isdetected by the polarization-mode dispersion quantity detecting unit 14.Further, in order to compensate the polarization-mode dispersion, theparameter setting circuit 215 controls the variable delay elements 18Aand 18B and the intensity variable elements 21A, 21B and 21C such thatthe fe (Hz) component intensity becomes the maximum.

Incidentally, the variable delay elements 18A and 18B, and the intensityvariable elements 21A, 21B and 21C are both controlled in FIG. 23.However, it is possible to use either one of these elements for thecontrol when sufficient characteristics can be obtained on the receivingside.

As above, it is possible to attain the same effects as the dispersioncompensation controlling apparatus 1B according to the thirdmodification of the first embodiment.

(B5) Description of a Fifth Modification of the First Embodiment

FIG. 24 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a fifth modification of the first embodiment isapplied, in which an object of a control can bechanged between beforesystem operation (before a start of system operation) and during systemoperation (after a start of system operation). A method of controlling apolarization-mode dispersion quantity takes the control mode 1, and thedetection form 1 is employed since a frequency provided for intensitydetection is one system.

The optical transmission system 40 shown in FIG. 24 is as well anoptical communication system with a transmission rate B (b/s) (forexample, 40 Gb/s, 10 Gb/s or the like) adopting time divisionmultiplexing. In the optical transmission system 40, an opticaltransmitter 22 as a transmitting terminal apparatus transmitting atransmission optical signal and an optical receiver 27 as a receivingterminal apparatus receiving the transmission optical signal areconnected over an optical transmission line (transmission fiber) 23, anda dispersion compensation controlling apparatus 39 and a switchchanging-over unit 38 are disposed on the optical receiving side.

In order to compensate polarization-mode dispersion generated in anoptical signal transmitted, the optical receiver 27 comprises apolarization-mode dispersion compensator 24, an optical splitting unit25 and an optical receiving unit 26. The polarization-mode dispersioncompensator 24 more efficiently compensates polarization-mode dispersiongenerated in the transmission optical signal according to a state of theoptical transmission line 3. The optical splitting unit 25 takes out apart of the transmission optical signal inputted to the receiving sideover the optical transmission line 3, and sends it out as monitor lightto the dispersion compensation controlling apparatus 39. The opticalreceiving unit 26 receives the transmission optical signal.

The dispersion compensation controlling apparatus 39 comprises,similarly to the dispersion compensation controlling apparatus 1according to the first embodiment, a photo receiver 28, a band-passfilter (fe BPF) 29, an intensity detector 30, a polarization-modedispersion quantity detecting unit 36 and a parameter setting circuit37. The dispersion compensation controlling apparatus 39 furthercomprises a compensation quantity optimization controlling unit 31 inorder to automatically perform a feedback control when polarization-modedispersion is compensated, a switch 38A switching an output of anintensity detector 30A between before and during system operation, and aswitch 38A′ operating in association with the switch 38A.

In this embodiment, a term “dispersion” is used to mean“polarization-mode dispersion”, as well, the dispersion compensationcontrolling apparatus 39 thus represents “polarization-mode dispersioncompensation controlling apparatus 39”.

Here, the photo receiver 28, the band-pass filter 29 and the intensitydetector 30 are similar to the photo receiver 11, the band-pass filter12 and the intensity detector 13, respectively, according to the firstembodiment, further descriptions of which are thus omitted. Thepolarization-mode dispersion quantity detecting unit 36 and theparameter setting circuit 37 are similar to the polarization-modedispersion quantity detecting unit 14 and the parameter setting circuit15, respectively, which function as a polarization-mode dispersioncontrolling unit 90 d. Since a frequency provided for intensitydetection is one system, here is employed the detection form 1.

The switch 38A drives the polarization-mode dispersion quantitydetecting unit 36 before operation of the optical transmission system 40in order to determine the optimum value of parameter information showinga polarization-mode dispersion compensation quantity, while driving thecompensation quantity optimization controlling unit 31 during theoperation in order to prevent fluctuation in the optimum value of theparameter information, which switches an output from the intensitydetector 30. Here, “before system operation” means a time when theoptical transmission system 40 is actuated or when the opticaltransmission system 40 is re-actuated if the polarization-modedispersion compensation control largely deviates from the optimum point,for example. The change-over control is performed by the switchchanging-over unit 38. The switch 38A′ inputs an output of thepolarization-mode dispersion detecting unit 36 or a phase comparingcircuit 33 to the parameter setting circuit 37, in association with theswitch 38A.

Incidentally, a switching controlling method to “optimize a compensationquantity in order to prevent fluctuations in the optimum value ofparameter information during operation” will be described later.

The compensation quantity optimization controlling unit 31 superimposesa predetermined low frequency signal set in advance on a parametersetting control signal outputted from the parameter setting circuit 37,and controls a parameter setting in the parameter setting circuit 37such that the above low frequency signal included in the intensity ofthe above first specific frequency component from the intensity detector30 beocmes zero, thereby optimizing a compensation quantity ofpolarization-mode dispersion of the above transmission optical signal.The compensation quantity optimization controlling unit 31 comprises aband-pass filter (f₀ BPF) 32, a phase comparing circuit 33, a lowfrequency oscillator 34 and a low frequency superimposing circuit 35.

The band-pass filter 32 extracts a low frequency signal component [f₀(Hz) component] included in the intensity of the first specificfrequency component [fe (Hz) component] detected by the intensitydetector 30. The phase comparing circuit 33 compares the low frequencysignal component extracted by the band-pass filter 32 with the lowfrequency signal from the low frequency generator 34 to detect adifference in phase, and controls the parameter setting in the parametersetting circuit 37 such that the low frequency signal componentextracted by the band-pass filter 32 becomes zero.

The low frequency superimposing circuit 35 superimposes a predeterminedlow frequency signal (f₀ signal) set in advance inputted from the lowfrequency oscillator 34 on the parameter setting control signaloutputted from the parameter setting circuit 37 to give a minutemodulation thereto, and sends out the modulated parameter settingcontrol signal to the polarization-mode dispersion compensator 24.

The compensation quantity optimization controlling unit 31 drives thepolarization-mode dispersion quantity detecting unit 36 to determine theoptimum value of parameter information showing a polarization-modedispersion quantity before system operation, while performing a controlto keep at all times a delay quantity Δτ of the optical transmissionline 3 at the optimum value during system operation.

A controlling method during system operation is as follows. Namely, thecompensation quantity optimization controlling unit 31 minutelymodulates a delay quantity Δτ be given by the polarization-modedispersion compensator 24 with a low frequency f₀ in order toautomatically fix the intensity of the first specific frequencycomponent in a baseband spectrum of a transmission optical signalinputted to the receiving side over the optical transmission line 23 tothe maximum value, so as to perform a tracking control in order to keepthe delay quantity Δτ at the optimum value at all times against a changewith time of the optical transmission line 23. As an example of thetracking control, in the feed-back control at the time of compensationof polarization-mode dispersion, the delay quantity Δτ is minutelyvaried (dithered) in the vicinity of the maximum point Δτ₀ to detect anew maximum point, thereby automatically determining it. Here, aprinciple of the feedback control by the compensation quantityoptimization controlling unit 31 will be described with reference toFIGS. 25(a) through (c) and FIGS. 26(a) through (g).

FIG. 25(a) shows a relationship between the delay quantity Δτ(transverse axis) and the fe (Hz) component intensity (vertical axis)after polarization-mode dispersion compensation, which schematicallyillustrates a situation [FIG. 25(c)] of changes in the fe (Hz) componentintensity when three kinds of low frequency signals (for example,signals of about 1 kHz) A, B and C shown in FIG. 25(b) are added to thedelay quantity Δτ (transverse axis). A signal waveform B shown in FIG.25(b) is a waveform changing with time at a frequency f₀ (Hz) in thecase where the parameter information is the maximum value. In this casewhere the delay quantity Δτ after polarization-mode dispersioncompensation is at the maximum value and the fe (Hz) component intensityis the maximum, the fe (Hz) component intensity changes with time at afrequency 2×f₀ as shown in FIG. 25(c), and contains no component of thefrequency f₀.

To the contrary, when the parameter information deviates from theoptimum value, that is, when the delay quantity Δτ deviates from theoptimum value as A or C deviates from a state of B shown in FIG. 25(b),the frequency f₀ (Hz) appears in a change with time of the fe (Hz)component frequency as shown in FIG. 25(c), moreover, codes of thecomponents of A and C are opposite (the phase is inverted).

Back to FIG. 24, the band-pass filter 32 detects a frequency f₀ (Hz)component from the fe (Hz) component intensity, and the parametersetting circuit 37 sets a delay quantity Δτ to be given by thepolarization-mode dispersion compensator 24 in such a direction that thefrequency component f₀ is cancelled. Accordingly, it is possible by suchfeedback to optimize a compensation quantity of polarization-modedispersion of the transmission optical signal. Incidentally, a directionof the change can be determined from a phase of the component of thefrequency f₀ (Hz) detected by the phase comparing circuit 33.

Whereby, the optical signal at a transmission rate B (b/s) transmittedfrom the optical transmitter 22 is transmitted to the optical receiver27 over the optical transmission line 23 in the optical transmissionsystem 40.

In this occasion, in the optical transmission system 40, the opticalsplitting unit 25 takes out a part of the optical signal transmittedover the optical transmission line 23, and the optical signal taken out(monitor light) is sent to the dispersion compensation controllingapparatus 39 in order to compensate polarization-mode dispersiongenerated in a transmitted optical signal.

In the dispersion compensation controlling apparatus 39, a state ofpolarization-mode dispersion generated in an optical signal transmittedover the optical transmission line 23 is monitored on the basis of theoptical signal taken out by the optical splitting unit 25, and thepolarization-mode dispersion compensator 24 is controlled according to aresult of the monitoring. First, before operation of the opticaltransmission system 40, the switch changing-over unit 38 changes overthe switch 38A and the switch 38A′ in order to drive thepolarization-mode dispersion quantity detecting unit 36 (contact pointsas shown in FIG. 24).

The optical signal taken out by the optical splitting unit 25 is thenreceived by the photo receiver 28, converted into an electric signal(O/E-converted), and inputted to the band-pass filter 29. In theband-pass filter 29, as having been described in the first embodiment,the first specific frequency component [fe (Hz) component] in a basebandspectrum of a transmission optical signal appropriately set according toa transmission rate or a signal waveform of the optical signal isdetected (specific frequency component detecting step).

Following that, an intensity of the first specific frequency componentdetected by the band-pass filter 29 is detected by the intensitydetector 30 (intensity detecting step). Further, by thepolarization-mode dispersion quantity detecting unit 36, apolarization-mode dispersion quantity of the above transmission opticalsignal is detected from the intensity of the first specific frequencycomponent detected by the intensity detector 30 by performing apredetermined first functional operation [that is, a functionaloperation using the above formulae (2) and (3)] (dispersion quantitydetecting step).

In order to compensate polarization-mode dispersion of the transmissionoptical signal, a parameter setting control signal for setting suchparameter information (delay quantity Δτ) as to cancel thepolarization-mode dispersion quantity detected by the polarization-modedispersion quantity detecting unit 36 is outputted from the parametersetting circuit 37 to the polarization-mode dispersion compensator 24disposed in the optical receiver 25 through the low frequencysuperimposing circuit 35 of the compensation quantity optimizationcontrolling unit 31. Incidentally, the low frequency superimposingcircuit 35 superimposes a low frequency signal (f₀ signal) from the lowfrequency oscillator 34 on the parameter setting control signal from theparameter setting circuit 37, and outputs it.

When the polarization-mode dispersion compensator 24 receives theparameter setting control signal, parameter information is set on thebasis of the control signal so as to compensate polarization-modedispersion generated in the optical signal transmitted over the opticaltransmission line 23. Following that, during operation of the opticaltransmission system 40, the switch changing-over unit 38 changes overthe switch 38A and the switch 38A′ (contact positions opposite to thoseshown in FIG. 24) in order to drive the compensation quantityoptimization controlling unit 31.

The optical signal taken out by the optical splitting unit 25 isinputted to the compensation quantity optimization controlling unit 31via the photo receiver 28, the band-pass filter 29 and the intensitydetector 30 in a similar manner described above. The compensationquantity optimization controlling unit 31 controls a parameter settingin the parameter setting circuit 37 such that a low frequency signalcomponent included in the intensity of the first specific frequencycomponent from the intensity detector 30 becomes zero, therebyoptimizing a compensation quantity of polarization-mode dispersion ofthe above transmission optical signal.

With the above structure, compensation is performed. An operation in thedispersion compensation controlling apparatus 39 at this time will befurther described with reference to FGIS. 26(a) through 26(g). Here,signal waveforms shown in FIGS. 26(a) through 26(g) correspond to signalwaveforms in portions denoted by reference characters (a) through (g) inthe dispersion compensation controlling apparatus 39 shown in FIG. 24.The waveforms shown in FIGS. 26(a) through 26(g) show a case where thedelay quantity Δτ deviates from the maximum point of the fe (Hz)component intensity toward the negative side (namely, in the case of Ain FIG. 25(b)).

First, a part of the transmission optical signal is split by the opticalsplitting unit 25 disposed in the rear stage of the polarization-modedispersion compensator 24, received by the photo receiver 28, and the fe(Hz) component is extracted by the band-pass filter 29. A signalwaveform denoted by (c) at an output of the band-pass filter 29 shown inFIG. 24 has, as shown in FIG. 26(c), an envelope in which the fe (Hz)component varies at a low frequency f₀ (Hz). This signal is convertedinto an intensity modulated signal at the low frequency fo by theintensity detector 30, and inputted to the compensation quantityoptimization controlling unit 31.

A component of the low frequency f₀ is extracted by the band-pass filter32 in the compensation quantity optimization controlling unit 31, and awaveform as shown in FIG. 26(e) is obtained. Further, a phase of thecomponent is compared with a phase of the f₀ (Hz) intensity componentfrom the low frequency oscillator 34 by the phase comparing circuit 33,and a signal according to a phase difference as shown in FIG. 26(g) isobtained. In this case, in the case of A shown in FIG. 25(b), a signalintensity shown in FIG. 26(g) increases proportionally as the delayquantity Δτ at the receiving terminal (optical receiving unit 26)increases.

In contrast, when the delay quantity Δτ deviates from the maximum pointof the fe (Hz) component intensity toward the positive side (namely, inthe case of C in FIG. 25(b)), the fe (Hz) component intensity decreasesas the delay quantity Δτ increases, so that a phase of an envelopevarying at the low frequency f₀ (Hz) shown in FIG. 26(c) is shifted by ahalf cycle (½ f₀). With this, a signal waveforms shown in FIGS. 26(d)and 26(e) are shifted with time by a half cycle, thus a code of a signal[refer to FIG. 26(g)] obtained as a result of the phase comparing isinverted.

Accordingly, the parameter setting circuit 37 detects a code of a signalobtained as a result of the phase comparing by the phase comparingcircuit 33 to determine whether the delay quantity Δτ is shifted to thepositive or negative direction, so that a parameter setting controlsignal for changing the delay quantity Δτ in such a direction as tocancel the f₀ (Hz) intensity modulated component in the Fe (Hz)component is generated, and outputted.

When receiving the parameter setting control signal, thepolarization-mode dispersion compensator 24 sets parameter informationon the basis of the control signal so as to compensate polarization-modedispersion generated in an optical signal transmitted over the opticaltransmission line 23.

As above, the dispersion compensation controlling apparatus 39 accordingto the fifth modification of the first embodiment detects an intensityof the first specific frequency component in a baseband spectrum in atransmission optical signal, and detects a polarization-mode dispersionquantity in the transmission optical signal from the detected intensityof the first specific frequency component by performing a predeterminedfirst functional operation, so that polarization-mode dispersiongenerated in the transmission optical signal is easily detected.

As above, the polarization-mode dispersion quantity is detected, andparameter information for compensating polarization-mode dispersiongenerated in the transmission optical signal is set on the basis of thedetected polarization-mode dispersion quantity, wherebypolarization-mode dispersion is compensated and deterioration of atransmission waveform of an optical signal is thus prevented, whichcontributes to a long-distance transmission of a high-speed opticalsignal. In addition, it is advantageous that, during system operation,the delay quantity Δτ is at all times kept at the optimum value againsta change with time of the optical transmission path 23.

Further, it is possible to optimize a compensation quantity ofpolarization-mode dispersion of a transmission optical signal by thecompensation quantity optimization controlling unit 31, andautomatically perform a feedback control when the polarization-modedispersion is compensated.

(B6) Description of a Sixth Modification of the First Embodiment

FIG. 27 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a sixth modification of the first embodiment isapplied, in which an object of a control is changeable between beforesystem operation and after start of system operation. A method ofcontrolling a polarization-mode dispersion quantity uses the detectionform 1 and the control mode 1.

The optical transmission system 40A shown in FIG. 27 is as well anoptical communication system with a transmission rate B (b/s) (forexample, 40 Gb/s, 10 Gb/s or the like) adopting time divisionmultiplexing. The optical transmission system 40A differs from theoptical transmission system 40 according to the fifth modification ofthe first embodiment in that a polarization-mode dispersion compensator24 is disposed in an optical transmitter 22A, other parts of which aresimilar to those of the optical transmission system 40 according to thefifth modification of the first embodiment.

Namely, in the optical transmission system 40A, an optical transmitter22A as a transmitting terminal apparatus transmitting a transmissionoptical signal and an optical receiver 27A as a receiving terminalapparatus receiving the transmission optical signal are connected overan optical transmission line (transmission fiber) 23, and a dispersioncompensation controlling apparatus 39 is disposed in the opticaltransmitter 22A. Incidentally, it is alternatively possible to disposethe dispersion compensation controlling apparatus 39 on the receivingside, and send back a result of a polarization state of an opticalsignal detected in the optical receiver 27A to the optical transmitter22A (not shown). As a method of sending-back in such case, a method ofpreparing another line with a low speed, or a method of multiplexinginformation on a transmission optical signal in the opposite direction,for example, is employable. In this modification, a term “dispersion” isused to mean “polarization-mode dispersion”, the dispersion compensationcontrolling apparatus 39 thus represents “polarization-mode dispersioncontrolling apparatus 39”.

Here, the optical transmitter 22A comprises a signal light source 41 andan optical modulator 42 along with a polarization-mode dispersioncompensator 24 in order to compensate polarization-mode dispersiongenerated in an optical signal to be transmitted. As this, when thepolarization-mode dispersion compensator 24 is disposed on thetransmitting side, it is possible to set the optical splitting ratio γ.

Light for reference is taken out at an optical splitting unit 25 betweenthe optical transmission line 23 and the optical receiver 27A, andinputted to the dispersion compensation controlling apparatus 39. In thedispersion compensation controlling apparatus 39, a parameter settingcircuit 37 outputs a parameter setting control signal for setting theabove parameter information to the polarization-mode dispersioncompensator 24 disposed in the transmitting terminal apparatustransmitting a transmission optical signal via a low frequencysuperimposing circuit 35, and a detection signal (output signal from aphase comparing circuit 33) obtained by comparing phases on thereceiving side (on the side of the optical receiver 27A) of the opticalsignal is sent back to the side of the optical transmitter 22A. As thismethod of sending-back, a method of preparing another line with a lowspeed, or a method of multiplexing information on a transmission opticalsignal in the opposite direction is employable. A compensation quantityoptimization controlling unit 31 superimposes a predetermined lowfrequency signal set in advance on a parameter setting control signaloutputted from the parameter setting circuit 37, and controls aparameter setting in the parameter setting circuit 37 such that theabove low frequency signal component included in the intensity of theabove first specific frequency component from an intensity detector(first intensity detecting unit) 30 becomes zero, thereby optimizing acompensation quantity of polarization-mode dispersion of the abovetransmission optical signal, as well.

Switches 38A and 38A′ in the dispersion compensation controllingapparatus 39 are switches driving a polarization-mode dispersionquantity detecting unit 36 in order to determine the optimum value ofparameter information showing a polarization-mode dispersioncompensation quantity before operation of the optical transmissionsystem 40A, and, after start of the operation, operating in associationto drive the compensation quantity optimization controlling unit 31 inorder to prevent fluctuations in the optimum value of the parameterinformation. This switching control is performed by a switchchanging-over unit 38 outside the dispersion compensation controllingapparatus 39.

Incidentally, the other parts denoted by the same reference charactershave the same functions as those of the optical transmission system 10according to the first embodiment, further descriptions of which arethus omitted.

With the above structure, the optical transmission system 40A operatesalmost in the similar manner to the optical transmission system 40 towhich the dispersion compensation controlling apparatus 39 according tothe fifth modification of the first embodiment described above isapplied.

Even when the polarization-mode dispersion compensator 24 is disposed inthe transmitting terminal apparatus, the compensation quantityoptimization controlling unit 31 minutely modulates the delay differenceΔτ between two polarization components or the intensity splitting ratioγ with a low frequency, thereby optimizing a compensation quantity ofpolarization-mode dispersion of a transmission optical signal.

As above, according to the dispersion compensation controlling apparatus39 according to the sixth modification of the first embodiment, it ispossible to attain the same advantages as the fifth modification of thefirst embodiment described above. In addition, it is possible to controla polarization direction such that the optical intensity splitting ratioγ is in the best state according to a state of the optical transmissionline 23 by controlling the polarization-mode dispersion compensator 24disposed in the optical transmitter 22A, so that polarization-modedispersion generated in the transmission optical signal is moreeffectively compensated. It is also possible to keep at all times thedelay quantity Δτ at the optimum value against a change with time of theoptical transmission line 23 during system operation.

(B7) Description of a Seventh Modification of the First Embodiment

FIG. 28 is a block diagram showing a structure of an otpicaltransmission system to which a dispersion compensation controllingapparatus according to a seventh modification of the first embodiment isapplied, in which an object of a control is changeable between beforesystem operation and after start of system operation. A method ofcontrolling a polarization-mode dispersion quantity adopts the detectionform 1 and the control mode 1, as well.

The optical transmission system 40B shown in FIG. 28 is as well anoptical communication system with a transmission rate B (b/s) (forexample, 40 Gb/s, 10 Gb/s or the like) adopting time divisionmultiplexing. The optical transmission system 40B differs from theoptical transmission system 40A according to the sixth modification ofthe first embodiment in that a Δτ compensator 24-A1 and a γ compensator24-A2 configuring a polarization-mode dispersion compensator 24Adisposed in an optical transimtter 22B are independently controlled, theother parts of which are almost similar to those of the opticaltransmission system 40A according to the sixth modification of the firstembodiment.

Namely, in the optical transmission system 40B, the optical transmitter22B as a transmitting terminal apparatus transmitting a transmissionoptical signal and an optical receiver 27A as a receiving terminalapparatus receiving the transmission optical signal are connected overan optical transmission line (transmission fiber) 23, and a dispersioncompensation controlling apparatus 39A and a switch changing-over unit38 are disposed on the side of the optical transmitter 22B.Incidentally, it is possible to dispose the dispersion compensationcontrolling apparatus 39A and the switch changing-over unit 38 on thereceiving side, and send back a result of a polarization state of anoptical signal detected in the optical receiver 27A to the opticaltransmitter 22B (not shown). As a method of sending-back in such case, amethod of preparing another line with a low speed, or a method ofmultiplexing information on a transmission optical signal in theopposite direction, for example, is employable. In this modification, aterm “dispersion” is used to mean “polarization-mode dispersion”, thedispersion compensation controlling apparatus 39A thus represents“polarization-mode dispersion compensation controlling apparatus 39A”.

The optical transmitter 22B comprises a signal light source 41 and anoptical modulator 42 along with the polarization-mode dispersioncompensator 24A in order to compensate polarization-mode dispersiongenerated in an optical signal to be transmitted. The polarization-modedispersion compensator 24A can set not only Δτ but also γ therein,comprises the Δτ compensator and the γ compensator, and canindependently control them.

Light for reference is taken out by an optical splitting unit 25 betweenthe optical transmission line 23 and the optical receiver 27A andinputted to the dispersion compensation controlling apparatus 39A, and acontrol signal is outputted to the Δτ compensator 24A-1 and the γcompensator 24A-2 in the above polarization-mode dispersion compensator24A.

The dispersion compensation controlling apparatus 39A comprises a photoreceiver 28, a band-pass filter (fe BPF) 29, an intensity detector 30, aswitch 38A, a polarization-mode dispersion quantity detecting unit 36along with a compensation quantity optimization controlling unit 31A anda parameter setting circuit 37. Further, the polarization-modedispersion quantity detecting unit 36 and the parameter setting circuit37 function as a polarization-mode dispersion controlling unit 90 d. Thephoto receiver 28, the band-pass filter 29, the intensity detector 30,the switch 38A and the polarization-mode dispersion quantity detectingunit 36 have similar or the same functions as those described above,further descriptions of which are thus omitted.

The compensation quantity optimization controlling unit 31A comprises aband-pass filter (f₁ BPF) 32A, a band-pass filter (f₂ BPF) 32B, phasecomparing circuits 33A and 33B, low frequency oscillators 34A and 34B,and low frequency superimposing circuits 35A and 35B, in order toindependently control the Δτ compensator 24A-1 and the γ compensator24A-2. Namely, the dispersion compensation controlling apparatus 39Aminutely modulates parameter setting control signals to the compensators24A-1 and 24A-2 with different frequencies f₁ and f₂, respectively.Incidentally, these have similar or the same functions and structures asthose described above.

In other words, the compensation quantity optimization controlling unit31A superimposes two low frequency signals (f₁ and f₂ signals) havingdifferent low frequency components as the above predetermined lowfrequency signals on the above parameter setting control signals,controls a setting of the splitting ratio γ of an optical intensity totwo polarization modes in the parameter setting circuit 37 such thateither one of the above low frequency signal components [f₁ (Hz), f₂(Hz) signal components] included in the intensity of the above firstspecific frequency component from the intensity detector 30 becomeszero, while controlling a setting of the delay quantity Δτ between theabove two polarization modes in the parameter setting circuit 37 suchthat the other one of the above two low frequency signal componentsincluded in the intensity of the above first specific frequencycomponent from the intensity detector 30 becomes zero.

In FIG. 28, the parameter setting circuit 37 outputs parameter settingcontrol signals for setting the above parameter information to thepolarization-mode dispersion compensator 24A disposed in the opticaltransmitter 22B transmitting a transmission optical signal via the lowfrequency superimposing circuits 35A and 35B, which comprises a Δτsetting circuit 37A setting Δτ and a γ setting circuit 37B setting γ.From the parameter setting circuit 37, detection signals (output signalsfrom the phase comparing circuit 33A and the phase comparing circuit33B) obtained by comparing phases on the receiving side (on the side ofthe optical receiver 27A) of an optical signal is sent back to the sideof the optical transmitter 22A. Incidentally, the low frequencysuperimposing circuits 35A and 35B superimpose low frequency sginals (f₁and f₂ signals) from the low frequency oscillators 34A and 34B on theparameter setting control signals from the setting circuits 37A and 37B,respectively, of the parameter setting circuit 37.

With the above structure, the optical transmission system 40B operatesalmost in the smilar manner to the optical transmission system 40A towhich the dispersion compensation controlling apparatus 39 according tothe fifth modification of the first embodiment described above isapplied.

Namely, in the dispersion compensation controlling apparatus 39A, anoptical signal taken out by the optical splitting unit 25 is inputted tothe polarization-mode dispersion quantity detecting unit 36 via thephoto receiver 28, the band-pass filter 29 and the intensity detector 30similarly to the case described above, before operation of the opticaltransmission system 40B.

A polarization-mode dispersion quantity of a transmission optical signalis detected by the polarization-mode dispersion quantity detecting unit36, and parameter setting control signals based on a result of thedetection are outputted from the setting circuits 37A and 37B of theparameter setting circuit 37 to the compensators 24A-1 and 24A-2 of thepolarization-mode dispersion compensator 24A disposed in the opticaltransimtter 22B via the low frequency superimposing circuits 35A and 35Bof the compensation quantity optimization controlling unit 31A.

When receiving the parameter setting control signals, thepolarization-mode dispersion compensator 24A sets parameter informationon the basis of the control signals so as to compensatepolarization-mode dispersion generated in an optical signal to betransmitted over the optical transmission line 23.

On the other hand, during operation of the optical transmission system40B, in the dispersion compensation controlling apparatus 39A, anoptical signal taken by the optical splitting unit 25 is inputted to thecompensation quantity optimization controlling unit 31A via the photoreceiver 28, the band-pass filter 29 and the intensity detector 30,similarly to the case described above.

The compensation quantity optimization controlling unit 31A controlsparameter settings in the setting circuits 37A and 37B of the parametersetting circuit 37 such that low frequency signal components included inthe intensity of the first specific frequency component from theintensity detector 30 become zero, thereby optimizing a compensationquantity of polarization-mode dispersion of the above transmissionoptical signal.

Namely, an fe (Hz) component intensity signal from the intensitydetector 30 is split, components of the low frequencies f₁ and f₂ (Hz)are extracted by the band-pass filters 32A and 32B with differentfrequencies f₁ and f₂, respectively, and phases of these low frequencycomponents and the f₁ and f₂ (Hz) components from the low frequencyoscillators 34A and 34B are compared by the phase comparing circuits 33Aand 33B, respectively. The setting circuits 37A and 37B of the parametersetting circuit 37 detect codes of signals obtained from results ofcomparing by the phase comparing circuits 33A and 33B as describedabove, thereby determining whether the delay quantity Δτ or the opticalintensity splitting ratio γ is shifted to either a positive or negativedirection, generate parameter setting control signals for changing thedelay quantity Δτ or the optical intensity splitting ratio γ in such adirection that the f₁ and f₂ (Hz) intensity modulated components in thefe (Hz) component are cancelled, and output the same.

When receiving the parameter setting control signals, the compensators24A-1 and 24A-2 of the polarization-mode dispersion compensator 24A setparameter information on the basis of the control signals so thatpolarization-mode dispersion generated in an optical signal to betransmitted over the optical transmission line 23 is compensated.

As above, according to the dispersion compensation controlling apparatus39A according to the seventh modification of the first embodiment ofthis invention, it is possible to attain the same advantages as thesixth modification of the first embodiment described above. In addition,it is advantageously possible to independently control the compensators24A-1 and 24A-2 of the polarization-mode dispersion compensator 24Adisposed in the optical transmitter 22B, thereby appropriatelycontrolling both the delay quantity Δτ and the optical intensitysplitting ratio γ.

(B8) Description of an Eighth Modification of the First Embodiment

As an optical system independently controlling the compensators 24A-1and 24A-2 of the polarization-mode dispersion compensator 24A disposedin the optical transmitter 22B, one shown in FIG. 29 is also possible.

FIG. 29 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to an eighth modification of the first embodiment ofthis invention is applied, in which an object of control is changeablebetween before system operation and after system operation. A method ofcontrolling a polarization-mode dispersion quantity uses the detectionform 1 and the control mode 1, as well.

The optical transmission system 40C is as well an optical communicationsystem with a transmission rate B (b/s) (for example, 40 Gb/s, 10 Gb/sor the like) adopting time division multiplexing. Namely, in the opticaltransmission system 40C, an optical transmitter 22B as a transmittingterminal transmitting a transmission optical signal and an opticalreceiver 27A as a receiving terminal apparatus receiving thetransmission optical signal are connected over an optical transmissionline (transmission fiber) 23, and a dispersion compensation controllingapparatus 39B, a switch changing-over unit 38 and switches 38D and 38Eare disposed on the optical transmitting side. Light for reference istaken out by an optical splitting unit 25 between the opticaltransmission line 23 and the optical receiver 27A and inputted to thedispersion compensation controlling apparatus 39B, and control signalsare outputted to a Δτ compensator 24A-1 and a γ compensator 24A-2 in apolarization-mode dispersion compensator 24A via switches 38D and 38E.Incidentally, it is alternatively possible to dispose the dispersioncompensation controlling apparatus 39B and the switch changing-over unit38 on the receiving side, and send back a result of a polarization stateof an optical signal detected in the optical receiver 27A to the opticaltransmitter 22B (now shown). As a method of sending-back in such case, amethod of preparing another line with a low speed, or a method ofmultiplexing information on a transmission optical signal in theopposite direction is employable. According to this modification, a term“dispersion” is used to mean “polarization-mode dispersion”, thedispersion compensation controlling apparatus 39 thus represents“polarization-mode dispersion controlling apparatus 39B”.

The dispersion compensation controlling apparatus 39B comprises a photoreceiver 28, a band-pass filter (fe BPF) 29, an intensity detector 30, aswitch 38A, a polarization-mode dispersion quantity detecting unit 36along with a compensation quantity optimization controlling unit 31B anda parameter setting circuit 37. Further, the polarization-modedispersion quantity detecting unit 36 and the parameter setting circuit37 function as a polarization-mode dispersion controlling unit 90 d.Incidentally, the photo receiver 28, the band-pass filter (fe BPF) 29,an intensity detector 30, the switch 38A and the polarization-modedispersion quantity detecting unit 36 have similar or the same functionsas those described above, further descriptions of which are thusomitted.

The compensation quantity optimization controlling unit 31B comprises,as shown in FIG. 29, a band-pass filter (f₀ BPF) 32, a phase comparingcircuit 33, a low frequency oscillator 34, low frequency superimposingcircuits 35A and 35B, a Δτ holding circuit 43, a γ holding circuit 44and switches 38B and 38C (on the output's side of the phase comparingcircuit 33).

The switches 38B and 38C change over a setting control on the opticalintensity splitting ratio γ and a setting control on the delay quantityΔτ according to a time, which are changed-over to a switch terminal “A”or a switch terminal “B” in association with each other according to acontrol signal outputted from the switch changing-over unit 38. Namely,when the control signal from the switch changing-over unit 38 is for theswitch terminal “A”, an output of the phase comparing circuit 33 isinputted to a γ setting circuit 37B and an output of the low frequencyoscillator 34 is inputted to the low frequency superimposing circuit35B, whereby a value of γ is controlled. To the contrary, when thecontrol signal from the switching changing-over unit 38 is for theswitch terminal “B”, an output of the phase comparing circuit 33 isinputted to a Δτ setting circuit 37A, and an output of the low frequencyoscillator 34 is inputted to the low frequency superimposing circuit35A, whereby a value of Δτ is controlled. The parameter setting circuit37 comprises the Δτ setting circuit 37A setting Δτ and the γ settingcircuit 37B setting γ.

The Δτ holding circuit 43 holds a value of the delay quantity Δτ beforeswitching-over, and outputs the value of the delay quantity Δτ when asetting control on the optical intensity splitting ratio γ is performed.The γ holding circuit 44 holds a value of the optical intensitysplitting ratio γ before switching-over, and outputs a value of theoptical intensity splitting ratio γ when a setting control on the delayquantity Δτ is performed.

The switch 38D is inputted a control signal thereto from the switchchanging-over unit 38 to select either an output of the Δτ holdingcircuit 43 or an output of the low frequency superimposing circuit 35Aand is changed-over to the selected one, and inputs it to the Δτcompensator 24A-1. Similarly, the switch 38E is inputted thereto acontrol signal from the switch changing-over unit 38 to select either anoutput of the γ holding circuit 44 or an output of the low frequencysuperimposing circuit 35B and is changed-over to the selected one, andinputs it to the γ compensator 24A-2.

The parts denoted by the same reference characters have similarfunctions and structures to those in the other modifications describedabove.

A controlling method according to this modification is as follows.Namely, in the control mode 1, an appropriate compensation quantity isdetermined from a value obtained by detection, and adjustment of thecompensators 24A-1 and 24A-2 of the polarization-mode dispersioncompensator 24A is changed-over according to a time and performedalternately.

Namely, during a predetermined time, a minute modulation is performed onthe delay difference Δτ with a low frequency, whereas during anotherpredetermined time, a minute modulation is performed on the intensitysplitting ratio γ with a low frequency, that is, two modulations areperformed alternately. In concrete, the switch changing-over unit 38shown in FIG. 29 interlocks the plural switches 38B and 38C tochange-over them between the switch terminal “A” and the switch terminal“B” at predetermined time intervals. At this time, control points of thecompensators 24A-1 and 24A-2 not controlled are held at positions beforechanged-over by the Δτ holding circuit 43 or the γ holding circuit 44. Areason why the controls are alternately performed with respect to timeis that even if the polarization-mode dispersion compensator 24Aoperates in order to compensate polarization-mode dispersion, it takes atime from about several minutes to a several hours until a change inpolarization-mode dispersion state of the optical transmission path 23actually appears.

Namely, the compensation quantity optimization controlling unit 31B ofthe dispersion compensation controlling apparatus 39B according to theeighth modification switches and alternately performs with respect totime the setting control on the splitting ratio γ to two polarizationmodes and the setting control on the delay quantity Δτ between the twoplarization modes.

Whereby, controlls on the compensators 24A-1 and 24A-2 are performedindependently.

With the above structure, the optical transmission system 40C operatesin almost the similar manner to the optical transmission system 40 towhich the dispersion compensation controlling apparatus 39 according tothe first embodiment described above.

Namely, before system operation, the switch changing-over unit 38switches an output signal of the intensity detector 30 to thepolarization-mode dispersion quantity detecting unit 36 to determine theoptimum value of the parameter information showing a polarization-modedispersion compensation quantity.

On the other hand, during system operation, the switch changing-overunit 38 switches an output signal of the intensity detector 30 to theband-pass filter 32 to drive the compensation quantity optimizationcontrolling unit 31B in order to prevent fluctuations in the optimumvalue of the parameter information.

The switch changing-over unit 38 switches the plural switches 38B and38C at predetermined time intervals, and in the compensators 24A-1 and24A-2 of the polarization-mode dispersion compensator 24 a disposed inthe optical transmitter 22B, minute modulations are alternatelyperformed with respect to time on the delay difference Δτ to and theintensity splitting ratio γ of the two polarization components with alow frequency.

As above, according to the dispersion compensation controlling apparatus39B of the eighth modification of the first embodiment of thisinvention, it is possible to attain the similar advantages to the caseof the sixth modification of the first embodiment described above. Inaddition, it is advantageously possible to decrease a load of thecontrols as compared with a case of simultaneous controls by switchingwith respect to time the controls on the compensators 24A-1 and 24A-2 ofthe polarization-mode dispersion compensator 24A disposed in the opticaltransmitter 22B.

(B9) Description of a Ninth Modification of the First Embodiment

FIG. 30 is a block diagram showing a structure of an opticaltransmission system according to a ninth modification of the firstembodiment of this invention. As a method of controlling apolarization-mode dispersion, here are used the detection form 1 and thecontrol mode 1.

The optical transmission system 50 shown in FIG. 30 is as well anoptical communication system with a transmission rate B (b/s) (forexample, 40 Gb/s, 10 Gb/s or the like) adopting time divisionmultiplexing. In the optical transmission system 50, an opticaltransmitter 52 as a transmitting terminal apparatus transmitting atransmission optical signal and an optical receiver 57 as a receivingterminal apparatus receiving the transmission optical signal areconnected over an optical transmission line (transmission fiber) 53, andsignal light is split by an optical splitting unit 55 on the receivingside, one of which is inputted to the optical receiver 57 while theother of which is inputted to a dispersion quantity detecting apparatus51. The dispersion quantity detecting apparatus 51 will be describedlater. Incidentally, in this modification, a term “dispersion quantitydetection” is used to mean “polarization-mode dispersion detection” aswell, the dispersion quantity detecting apparatus 51 thus represents“polarization-mode dispersion quantity detecting apparatus 51”.

In the optical transmission system 50, a signal light source 62 and anoptical modulator 63 are disposed in the optical transmitter 52, alongwith a polarization-mode dispersion compensator 54 for artificiallygiving polarization-mode dispersion to an optical signal to betransmitted. Since the polarization-mode dispersion compensator 54 is onthe transmitting side, it is possible to set the optical intensitysplitting ratio γ.

A sweep controlling unit 56 is disposed on the optical transmittingside. The sweep controlling unit 56 largely sweeps and controlsparameters showing the above polarization-mode dispersion quantity to beartificially given by the polarization-mode dispersion compensator 54 inorder to obtain the optimum value of parameter information showing apolarization-mode dispersion quantity, before operation of the opticaltransmission system (namely, when the optical transmission system 50 isactuated, or when the optical transmission system 50 is re-actuated ifthe polarization-mode dispersion compensating control largely deviatesfrom the optimum point). Namely, there is provided the sweep controllingunit 56 largely sweeping and controlling parameters showing the abovepolarization-mode dispersion quantity to be given by thepolarization-mode dispersion compensator 54 when the system is actuatedor when the system is re-actuated.

In the optical transmission system 50, a dispersion quantity detectingapparatus 51 is disposed in the optical receiver 57. The dispersionquantity detecting apparatus 51 monitors a state of polarization-modedispersion generated in an optical signal transmitted over the opticaltransmission line 53 on the basis of an optical signal taken out by theoptical splitting unit 55, which comprises, as shown in FIG. 30, a photoreceiver 58, a band-pass filter (fe BPF) 59, an intensity detector 60and a polarization-mode dispersion quantity detecting unit 61.Incidentally, these parts have similar functions and structures to thoseof the first embodiment described above.

In the optical transmission system 50, an optical signal at atransmission rate B (b/s) transmitted from the optical transmitter 52 istransmitted to the optical receiver 57 over the optical transmissionline 53. At this time, in the optical transmitter 52, polarization-modedispersion is artificially given to the optical signal by thepolarization-mode dispersion compensator 54 under a control of the sweepcontrolling unit 56.

Following that, a part of the optical signal transmitted over theoptical transmission line 53 is taken out by the optical splitting unit55, and the optical signal taken out (monitor light) is sent to thedispersion quantity detecting apparatus 51. In the dispersion quantitydetecting apparatus 51, a state of polarization-mode dispersiongenerated in the optical signal transmitted over the opticaltransmission path 53 is monitored on the basis of the optical signaltake out by the optical splitting unit 55.

In the above structure, a sweep control is performed before systemoperation. First, a parameter (at least either the delay quantity Δτ orthe optical intensity splitting ratio γ) showing a polarization-modedispersion quantity to be given to an optical signal by thepolarization-mode dispersion compensator 54 is swept and controlled in awide range. For example, the delay quantity Δτ is swept in a range fromΔτ₁ to Δτ₂, and the optical splitting ratio γ is swept in a range from 0to 1.

In the dispersion quantity detecting apparatus 51, an intensity of thefirst specific frequency component [fe (Hz) component] in a basebandspectrum in the optical signal artificially given the abovepolarization-mode dispersion is detected by the photo receiver 58, theband-pass filter 59, and the intensity detector 60, and apolarization-mode dispersion quantity of the transmission optical signalis detected by the polarization-mode dispersion quantity detecting unit61 in a manner similar to the above.

Here, the sweep control before operation of the optical transmissionsystem 50 will be described with reference to FIGS. 31(a) and 31(b).FIG. 31(a) shows a change in intensity of the first specific frequencycomponent when the delay quantity Δτ is swept in a range from Δτ₁ toΔτ₂. FIG. 31(b) shows a change in intensity of the first specificfrequency component when the optical splitting ratio γ is swept in arange from 0 to 1. As seen from FIGS. 31(a) and 31(b), the intensity ofthe first specific frequency component is the maximum when the delayquantity is Δτ₀ or the optical intensity splitting ratio is γ₀.

In consequence, a delay quantity Δτ₀ or an optical intensity splittingratio y₀ is determined as the optimum value of the parameter informationshowing a polarization-mode dispersion quantity, an operating point ofthe polarization-mode dispersion compensator 54 is such set as the delayquantity Δτ=Δτ₀ or the optical intensity splitting ratio γ=γ₀, and theoperation of the optical transmission system 50 is started.

Incidentally, it is possible to perform a tracking control duringoperation of the optical transmission system 50 in order to keep thedelay quantity Δτ or the optical intensity splitting ratio γ at theoptimum values at all times against a change with time of the opticaltransmission path 53. As an example of the tracking control, it ispossible to use a method of automatically performing a feedback controlwhen polarization-mode dispersion is compensated, as described above inthe fifth modification of the first embodiment. And, as shown in FIGS.31(a) and 31(b), the delay quantity Δτ or the optical intensitysplitting ratio γ is minutely varied (dithered) in the vicinity of themaximum point Δτ₀ or γ₀, thereby detecting a new maximum point.

As above, according to the dispersion quantity detecting apparatus ofthe ninth modification of the first embodiment of this invention, thesweep controlling unit 56 largely sweeps and controls a parametershowing the above polarization-mode dispersion quantity to beartificially given by the polarization-mode dispersion compensator 54before operation of the optical transmission system 50, whereby theoptimum value of parameter information showing a polarization-modedispersion quantity is determined.

Meanwhile, in the optical transmission system 50 shown in FIG. 30, thepolarization-mode dispersion compensator 54 is disposed in the opticaltransmitter 52. It is alternatively possible that the polarization-modedispersion compensator 54 is disposed in another position where, forexample, the optical receiver 57, a linear repeater (not shown) or thelike is disposed, and a similar control is performed.

(C) Description of a Second Modification

The method of controlling a polarization-mode dispersion quantity in thefirst embodiment and the modifications thereof described above is in thecontrol mode (control mode 1) using the first function. This method maybe performed in another way.

FIG. 32 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a second embodiment of this invention is applied(the same structure is also adopted in a first modification of thesecond embodiment to be described later). The optical transmissionsystem 210C shown in FIG. 32 is as well an optical communication systemwith a transmission rate B (b/s) (for example, 10 Gb/s or the like)adopting time division multiplexing. In the optical transmission system210C, an optical transmitter 2 as a transmitting terminal apparatustransmitting a transmission optical signal and an optical receiver 207 aas a receiving terminal apparatus receiving the transmission opticalsignal are connected over an optical transmission line (transmissionfiber) 3, and a dispersion compensation controlling apparatus 225 isdisposed on the receiving side. Incidentally, a term “dispersion” isgenerally used to mean “chromatic dispersion”. In the second embodiment,the term “dispersion” is used to mean “polarization-mode dispersion”,and “polarization-mode dispersion compensation controlling apparatus225” is mentioned as “PMD compensation controlling unit” in FIG. 32.

The optical receiver 207 a comprises a polarization controller 4B, aninter-polarization-mode variable delay unit 227, an optical splittingunit 5 and an optical receiving unit 6. The optical splitting unit 5 andthe optical receiving unit 6 have the same functions as those describedabove, further descriptions of which are thus omitted. Theinter-polarization-mode variable delay unit 227 gives a delay differenceΔτ between polarization modes to perform polarization-mode dispersioncompensation (Polarization-Mode Dispersion compensation), which isvariable. Enlarged diagrams of the polarization controller 4B and theinter-polarization-mode variable delay unit 227 are shown in FIG. 33.

The polarization controller 4B shown in FIG. 33 is used to adjust theaxis when a received optical signal is inputted to a fiber. Thepolarization controller 4B has wave plates [a ¼ wave plate (λ/4 plate)4B-11 and a ½ wave plate (λ/2 plate) 4B-12] which can be driven from theoutside. The wave plates 4B-11 and 4B-12 are driven by actuators 4B-13,4B-14, respectively, receiving parameter setting control signals fromthe outside.

Optical intensity can be decomposed into two kinds of polarization-modecomponents α and β (radian). According to the second embodiment, usingvariability of the inter-polarization-mode variable delay unit 227,these polarization mode components α and β are directly and dynamicallycontrolled. An a control is performed at the ¼ wave plate 4B-11, while βcontrol is performed at the ½ wave plate 4B-12. In other words, afunctional operation is performed in terms of the optical intensitysplitting ratio γ in the first embodiment, wherein, so to speak, astatic (Static) side of the optical intensity is used. In thisembodiment, using, so to speak, a dynamic (Dynamic) side of the otpicalintensity, a control by adjustment of a polarization angle is performed.

Next, a controlling method according to this embodiment will bedescribed. This method is performed in a mode that the dispersioncompensation controlling apparatus 225 performs a feedback control on atleast either the polarization controller 4B or theinter-polarization-mode variable delay unit 227 disposed in the opticaltransmission line 3 such that the intensity of the first specificfrequency component detected by the intensity detector 13 becomes themaximum. Namely, not determining a control quantity using a function asin the first embodiment, but determining a control quantity byfeeding-back such that the detected intensity of the specific frequencybecomes the maximum. In order to discriminate this control mode from thecontrol mode 1 (control using the first function), this control modewill be referred to as a control mode 2 in the following description.

Hereinafter, as methods of controlling polarization-mode dispersion,there are a method in which the first functional operation is performedin terms of two variables γ and Δτ, and a method in which an optimumvalue control is performed on at least either α and β, or Δτ.

In order to perform a dynamic control in the control mode 2, theinter-polarization-mode variable delay unit 227 shown in FIG. 33comprises polarization beam splitters (PBS) 227 a and 227 d and anoptical attenuator 227 b. Namely, the inter-polarization-mode variabledelay unit 227 is configured as a device separating polarization-modecomponents by the polarization beam splitter 227 a, giving a delaydifference between the polarization mode components by the variableoptical delay 272 c, and multiplexing them. One of the components isdelayed by the variable optical delay 227 c through an optical fiber 229a and outputted to an optical fiber 229 b, while the other component issubjected to a loss by the optical attenuator 227 b such that opticallosses in both optical paths are equal, multiplexed by the polarizationbeam splitter 227 d still in an orthogonal state, and outputted.

As above, it is advantageously possible to not only decrease penalty bycontrolling using the inter-polarization-mode variable delay unit 227 ascompared with a case where an inter-polarization-mode fixed delay isused but also comply with fluctuations in polarization-mode dispersionquantity of the optical transmission path due to switching of bit rate,transmission distance, signal modulation format or the like. A delaydifference to be given by a variable optical delay can be changed by acontrol signal from the outside.

FIGS. 34(a) through 34(c) show examples of a variable optical delay pathaccording to the second embodiment of this invention. Each of theseoptical delay paths functions as the variable optical delay 227 c, inwhich an optical signal is once taken out in the air, given a delaydifference, and put back again to a fiber. The optical fibers 229 a and229 b correspond to optical fibers at an input and an output of thevariable optical delay 227 c shown in FIG. 33. FIG. 34(a) shows a methodof using a reflecting mirror 228 c, FIG. 34(b) shows a method of using acorner cube 228 d, and FIG. 34(c) shows a method of using a method orthe like moving the fiber 229 b. Incidentally, in each of the drawings,reference characters 228 a and 228 b denote collimator lenses.

FIG. 35 shows an example of a structure of anotherinter-polarization-mode variable delay unit according to the secondembodiment of this invention. In an inter-polarization-mode delayelement 230 shown in FIG. 35, a plurality of polarization maintainingfibers (PMF) 230 c ₁, 230 c ₂, 230 c ₃, . . . having differentpolarization-mode dispersion values are arranged in parallel, andoptical switches 230 a and 230 b are disposed on the input's side andthe output's side thereof. These optical switches 230 a (or 230 b) leadan inputted optical signal to a corresponding PMF 230 c ₁, 230 c ₂, 230c ₃, . . . according to a control signal from the outside. The PMFs 230c ₁, 230 c ₂, 230 c ₃, . . . have polarization-mode dispersion valuesΔτ₁, Δτ₂, Δτ₃, . . . , respectively, where Δτ₁<Δτ₂<Δτ₃. Further,according to a control signal inputted according to a polarizationdispersion quantity of an inputted optical signal, a PMF close to avalue thereof is selected. In order to increase a variable quantity or avariable accuracy of polarization dispersion, it is only necessary toprepare a larger number of PMFs therefor. Namely, theinter-polarization-mode delay element (inter-polarization-mode delayunit) 230 is configured as a device in which a plurality of polarizationmaintaining fibers having different polarization dispersion values arearranged in parallel, and the polarization maintaining fiberstransmitting an optical signal are switched by the optical switch 230 a(or 230 b) according to a polarization-mode dispersion quantity of theoptical transmission line 3, and the inter-polarization-mode delay unit230 is configured with polarization maintaining fibers. Further, theinter-polarization-mode delay unit 230 is configured with aninter-polarization-mode variable delay unit in a state where a delayquantity is fixed.

Again back to FIG. 32, the dispersion compensation controlling apparatus225 shown in FIG. 32 is a dispersion compensation controlling apparatuscorresponding to the dispersion compensation controlling apparatus 1(1A, 1B, 39, 39A, 39B or the like) according to the first embodiment,which comprises a photo receiver 11, a band-pass filter 12, an intensitydetector 13, and an α·β·Δτ_(c) setting circuit 226. The photo receiver11, the band-pass filter 12 and the intensity detector 13 have the samefunctions as those described above, further descriptions of which arethus omitted.

The α·β·Δτ_(c) setting circuit 226 performs an appropriate control froma signal inputted from the intensity detector 13 to control thepolarization controller 4B in the optical receiver 207 a. Thispolarization-mode controlling function is achieved by a CPU or the like.

From the above, a flow of a signal is as follows. Namely, an opticalsignal at B (b/s) transmitted from the optical transmitter 2 issubjected to waveform deterioration due to polarization-mode dispersionof Δτ_(F) (ps/km^(1/2)) in the optical transmission line 3, and inputtedto the optical receiver 207 a. In the polarization controller 4B in theoptical receiver 207 a, an axis of the optical signal is adjusted by the¼ wave plate 4B-11 and the ½ wave plate 4B-12, given a delay differenceΔτ_(c) between polarization modes, and compensated its polarization-modedispersion using the inter-polarization-mode variable delay unit 227which can change Δτ_(c). A part of the optical signal compensated issplit by the optical splitting unit 5. One of the split signal isO/E-converted by the photo receiver 11 in the dispersion compensationcontrolling apparatus 225, a frequency component of fe (Hz) is extractedby the band-pass filter 12, and an intensity thereof is detected by theintensity detector 13. The other of the split signal is inputted to theoptical receiver 6.

In this intensity detection, a component intensity of fe=B/2 (Hz) thatis a half of the bit rate is detected, and three parameters of anazimuth angle α of the λ/4 plate, an azimuth angle β of the λ/2 plateand Δτ_(c) are controlled by the α·β·Δτ_(c) setting circuit 226 suchthat this intensity becomes the maximum. Incidentally, although here aredisposed these polarization dispersion compensating devices in thereceiving terminal, it is alternatively possible to dispose them in thetransmitting terminal or an optical repeater, detect a polarization-modedispersion quantity at the receiving terminal and feedback-control thepolarization compensation devices. The system using the fe=B/2 (Hz)component intensity can be adopted to not only NRZ waveforms but also RZwaveforms or OTDM waveforms.

As above, since an inter-polarization-mode variable delay element isused in this embodiment, waveform deterioration due to polarization-modedispersion is decreased, and it is possible to comply with fluctuationsin polarization-mode dispersion quantity of the optical transmissionline due to a switching of bit rate, transmission distance, signalmodulation format or the like. When an inter-polarization-mode fixeddelay element is used, it is sufficient to give a reference inconsideration of system conditions with respect to a design of a fixeddelay quantity thereof.

(C1) Description of a First Modification of the Second Embodiment

According to the second embodiment, it is possible to provide a functionof switching between before system operation (occasionally referred toas before system operation or a mode setting an initial value) andduring system operation (occasionally referred to as during systemoperation or a normal use mode). In the same structure as the one shownin FIG. 32, a method of controlling an initial setting mode is in thecontrol mode 2 to perform polarization-mode dispersion compensation.Since a frequency provided for intensity detection is in one system, itmeans that the detection form 1 is employed. In this modification, aterm “dispersion” is used to mean “polarization-mode dispersion”.

The method is that α, β and Δτ_(c) that are three parameters to be givenby the λ/4 and λ/2 plates and the inter-polarization-mode variable delayelement are changed in the full range at a sufficiently small pitch, andan intensity of the frequency fe (Hz) component is detected for everycombination of these three parameters. A combination of the α, β andΔτ_(c) making the frequency fe (Hz) components the maximum is obtainedas a result. At that time, optical waveform deterioration aftercompensation is the minimum so that the α, β and Δτ_(c) are set to thosevalues when the system is started.

When a tracking control is started without performing the initialsetting mode, there is a possibility that a total polarization-modedispersion quantity after polarization-mode dispersion compensation islarger than one time slot at a point of start of the control. In suchcase, in the characteristic curve of Δτ versus fe (=B/2) (Hz) componentintensity in FIG. 13, the frequency fe (Hz) component intensityincreases with increasing polarization-mode dispersion quantity, so thatwaveform deterioration increases due to a maximum value control on thefrequency fe (Hz) component. In contrast, by performing the initialsetting mode, the frequency fe (Hz) component is the maximum when thepolarization-mode dispersion quantity Δτ_(T) after compensation is theminimum so long as the transmission path polarization-mode dispersionquantity Δτ_(F) does not exceed one time slot, so that the trackingcontrol can be started from a correct position.

FIGS. 36 and 37 show a control flowchart for realizing polarization-modedispersion compensation according to the second embodiment of thisinvention (incidentally, this flowchart will be also used in third andfourth embodiments). First, the dispersion compensation controllingapparatus (mentioned as a PMD compensation controlling unit in FIG. 32)225 starts a program (Step A1), and performs a control of the initialsetting mode when system operation is started (Step A2). Next, adirection of a change of α, β and Δτ_(c) is initialized (Step A3), andthe dispersion controlling apparatus 225 increases a by a constant pitchα1 with β and Δτ_(c) being fixed to set a value of α (Step A4). Further,by changing a it is determined whether the fe (Hz) component intensityincreases or not (Step A5). When the fe (Hz) component intensityincreases, YES route is taken, and the dispersion compensationcontrolling apparatus 225 changes α at the same pitch α1 in the samedirection. When the fe (Hz) component intensity decreases, NO route istaken, and the dispersion compensation controlling apparatus 225, in theopposite direction (Step A6), changes α at the same pitch α1, so that anew α is set in either case (Step A7). Again, it is determined whetherthe fe (Hz) component intensity increases or not (Step A8). When the fe(Hz) component intensity increases, YES route is taken, and thedispersion compensation controlling apparatus 225 changes α at the samepitch α1. The α changing operation is continued until the fe (Hz)component intensity decreases (Step A7, Step A8). When the fe (Hz)component intensity does not increase at Step A8, the dispersioncompensation controlling apparatus 225 takes NO route, and onceterminates the control on α (first control mode).

Next, the dispersion compensation controlling apparatus 225 performs acontrol on β in the similar manner. Namely, at Step A9, the dispersioncompensation controlling apparatus 225 increases β by a constant pitch βto set β, and at Step A10 determines whether or not the fe (Hz)component frequency increases by changing β. When the fe (Hz) componentfrequency increases, the dispersion compensation controlling apparatus225 takes YES route via a point denoted by {circle around (1)} in FIG.36, and changes β at the same pitch β1 in the same direction (Step A12).On the contrary, when the fe (Hz) component intensity decreases at StepA10, the dispersion compensation controlling apparatus 225 takes NOroute, and, in the opposite direction (Step A11), changes β at the samepitch β1 to set a new β in the similar manner (Step A12). At Step A13,the dispersion compensation controlling apparatus 225 again determineswhether or not the fe (Hz) component intensity increases, repeats theboth of Steps A12 and A13, and performs a control such that thefrequency fe (Hz) component intensity becomes the maximum. When thefrequency fe (Hz) component intensity does not increase at Step A13, thedispersion compensation controlling apparatus 225 takes NO route, andonce terminates the control on β (second control mode).

Finally, the dispersion compensation controlling apparatus 225 performsa control on Δτ in the similar manner. Namely, at Step A14, thedispersion compensation controlling apparatus 225 increases Δτ by aconstant pitch Δτ₁ to set Δτ, and determines whether or not the fe (Hz)component intensity increases by changing Δτ (step A15). When the fe(Hz) component intensity increases, the dispersion compensationcontrolling apparatus 225 takes YES route, and changes Δτ at the samepitch Δτ₁ in the same direction (Step A17). On the contrary, when the fe(Hz) component intensity decreases at Step A15, the dispersioncompensation controlling apparatus 225 takes NO route, and in theopposite direction (Step A16), changes Δτ at the same pitch Δτ₁ to set anew Δτ (Step A17). At Step A18, the dispersion compensation controllingapparatus 225 again determines whether or not the fe (Hz) componentintensity increases. When the fe (Hz) component intensity increases, thedispersion compensation controlling apparatus 225 takes YES route, andrepeats the both of Steps A17 and A18. When the frequency fe (Hz)component intensity does not increase at Step A18, the dispersioncompensation controlling apparatus 225 takes NO route, once terminatesthe control on Δτ (third control), and moves back to Step A3 in FIG. 36via a point denoted by {circle around (2)} in FIG. 36.

As above, one control cycle is finished, and again the next controlcontinues from α in the similar manner. Namely, the polarization-modedispersion controlling unit (dispersion compensation controllingapparatus 225) performs a control in a first control mode in which thepolarization-mode dispersion controlling unit changes any one of anazimuth angle of the ¼ wave plate 4B-11, an azimuth angle of the ½ waveplate in the polarization controller 4B and a delay quantity betweenpolarization modes of the inter-polarization-mode delay unit 227 suchthat the intensity of the first specific frequency component becomes themaximum while fixing the remaining parameters among the above azimuthangles and the delay quantity between polarization modes. After thefirst control mode, the dispersion-mode controlling unit performs acontrol in a second control mode in which the polarization-modedispersion controlling unit changes either one of the remaining controlparameters such that the intensity of the first specific frequencycomponent becomes the maximum while fixing the parameter having beenfirst changed and the other one of the remaining control parameters.Finally, the polarization-mode dispersion controlling unit performs acontrol in the third control mode in which the polarization-modedispersion controlling unit changes the other one of the remainingparameters such that the intensity of the first specific frequencycomponent becomes the maximum, while fixing the control parameter havingbeen first changed and the one of the remaining control parameters.

As above, since a tracking control is performed during system operation,it is possible to capture the maximum values of changing α, β andΔτ_(c), so as to follow fluctuations in these parameters due to a changein external environment such as temperature.

Since the initial setting mode before system operation is performed asabove, it is advantageously possible to obtain the optimum state evenfrom a start of system operation. In addition, it is also advantageouslypossible to normally perform the tracking control during operation.

FIG. 38 shows a control flowchart for realizing polarization-modedispersion compensation according to the second embodiment of thisinvention (incidentally, this flowchart will be also used in the thirdand fourth embodiments). Although the initial setting mode at the timeof start of system operation is similar to the flowchart shown in FIGS.36 and 37 (Steps B1 through B3), here is featured that a control isswitched on α→β→Δτ_(c)→α→ . . . with each change at each step in thetracking control during system operation.

Namely, the dispersion compensation controlling apparatus 225 performsthe initial setting (Step B1 through Step B3), and sets a value of α atStep B4. By changing α, the dispersion compensation controllingapparatus 225 determines whether or not the fe (Hz) component intensityincreases (Step B5). When the fe (Hz) component intensity increase here,the dispersion compensation controlling apparatus 225 takes YES route.On the contrary, when the fe (Hz) component intensity decreases, thedispersion compensation contorlling apparatus 225 takes NO route, andchanges a value of α in the opposite direction (Step B6). Whereby, afourth control mode is performed (Step B4 through Step B6).

Further, the dispersion compensation controlling apparatus 225 changes avalue of β at a pitch β1 (Step B7), and determines at Step B8 whether ornot the fe (Hz) component intensity increases. When the fe (Hz)component intensity increases, the dispersion compensation controllingapparatus 225 takes YES route. On the contrary, when the fe (Hz)component intensity decreases, the dispersion compensation controllingapparatus 225 takes NO route, and changes a value of β in the oppositedirection (Step B9). Whereby, a fifth control mode is performed (Step B7through B9).

Finally, the dispersion compensation controlling apparatus 225 changes avalue of Δτ at a pitch Δτ (Step B10), and at Step B11 determines whetheror not the fe (Hz) component intensity increases. The fe (Hz) componentintensity increases, the dispersion compensation controlling apparatus225 takes YES route. On the contrary, when the fe (Hz) componentintensity decreases, the dispersion compensation controlling apparatus225 takes NO route, and changes a value of Δτ in the opposite direction(Step B12). Whereby, a sixth control mode is performed (Step B10 throughStep B12).

The controlling method shown in FIG. 38 has a faster convergence to theoptimum point than the controlling method shown in FIGS. 36 and 37.Namely, the polarization-mode dispersion controlling unit (dispersioncompensation controlling apparatus 225) performs a control in a fourthcontrol mode in which the polarization-mode dispersion controlling unitchanges any one of an azimuth angle of the ¼ wave plate 4B-11, anazimuth angle of the ½ wave plate 4B-12 in the polarization controller4B and a delay quantity between polarization modes of theinter-polarization-mode delay unit 227 such that the intensity of thefirst specific frequency component increases while fixing the remainingcontrol parameters among the above azimuth angles and the delay quantitybetween polarization modes. After the fourth control, thepolarization-mode dispersion controlling unit performs a control in afifth control mode in which the polarization-mode dispersion controllingunit changes one of the remaining control parameters such that theintensity of the first specific frequency component increases whilefixing the control parameter having been first changed and the other oneof the remaining control parameters. Finally, the polarization-modedispersion controlling unit performs the sixth control mode in which thepolarization-mode dispersion controlling unit changes the other one ofthe remaining control parameters such that the intensity of the firstspecific frequency component increases while fixing the controlparameter having been first changed and the one of the remaining controlparameters. After that, the polarization-mode dispersion controllingunit repeatedly executes the fourth control mode, the fifth control modeand the sixth control mode described above until the intensity of thefirst specific frequency component becomes the maximum.

As above, since the initial setting mode before system operation isperformed as shown in FIGS. 36 through 38, it is advantageously possibleto obtain the optimum state even during system operation, and normallyperform the tracking control during operation.

(C2) Description of a Second Modification of the Second Embodiment

FIG. 39 is a block diagram showing a structure of an opticaltransmission system according to a second modification of the secondembodiment of this invention, in which a control on aninter-polarization-mode variable delay is performed in analogue, and acontrol using the control mode 2 is performed. The optical transmissionsystem 210D shown in FIG. 39 is as well an optical communication systemwith a transmission rate B (b/s) (for example, 10 Gb/s or the like)adopting time division multiplexing. In the optical transmission system210D, as shown in FIG. 39, an optical transmitter 2 and an opticalreceiver 207 a are connected over an optical transmission line(transmission fiber) 3, and a dispersion compensation controllingapparatus 225 a is disposed on the receiving side. The opticaltransmitter 2, the optical receiver 207 a and the optical transmissionline 3 are the same as those described above, further descriptions ofwhich are thus omitted. Incidentally, in this modification, a term“dispersion” is used to mean “polarization-mode dispersion”.

The dispersion compensation controlling apparatus 225 a comprises, aphoto receiver 11, a band-pass filter 12 and an intensity detector 13,along with band-pass filters (f₁, f₂, f₃ BPF) 232A, 232B and 232C, phasecomparing circuits 233A, 233B and 233C, an α setting circuit 237A, a βsetting circuit 237B, a Δτ_(c) setting circuit 237C, low frequencysuperimposing circuits 235A, 235B and 235C, and low frequency generators234A, 234B and 234C. The photo receiver 11, the band-pass filter 12 andthe intensity detector 13 have the same functions as those describedabove, further descriptions of which are thus omitted.

The band-pass filters 232A, 232B and 232C extract low frequency signalcomponents [f₁, f₂, f₃ (Hz) components] included in the intensity of thefirst specific frequency component detected by the intensity detector13. The phase comparing circuit 233A compares the low frequency signalcomponent extracted by the band-pass filter 232A with a low frequencysignal from the low frequency generator 234A to detect a difference inphase, and controls a parameter setting in the a setting circuit 235Asuch that the low frequency signal component extracted by the band-passfilter 232A becomes zero. Similarly, input sides of the phase comparingcircuits 233B and 233C correspond to the band-pass filters 232B and232C, while the output sides of the same correspond to the β settingcircuit 237B and the Δ_(c) setting circuit 237C. Further, the α settingcircuit 237A, the β setting circuit 237B and the Δ_(c) setting circuit237C perform appropriate controls from signals inputted from the phasecomparing circuits 233A, 233B and 233C to determine values of α, β andΔτ, respectively.

Further, the low frequency superimposing circuits 235A and 235Bsuperimposes predetermined low frequency signals (f₁ signal and f₂signal) inputted from the low frequency oscillators 234A and 234B on anα setting control signal and a β setting control signal outputted fromthe a setting circuit 237A and the β setting circuit 237B, respectively,to give minute modulation thereto, and send the modulated parametersetting control signals to the polarization controlling unit 4B.Similarly, the low frequency superimposing circuit 235C superimposes apredetermined low frequency signal (f₃ signal) set in advance inputtedfrom the low frequency oscillator 234C on a Δτ_(c) setting controlsignal outputted from the Δτ_(c) setting circuit 237C to give minutemodulation thereto, and sends out the modulated parameter settingcontrol signal to an inter-polarization-mode variable delay unit 227.

From the above, the optical transmission system 210D is provided with acompensation quantity optimization controlling unit 241 whichsuperimposes predetermined low frequency signals set in advance oncontrol signals to be outputted from the polarization-mode dispersioncontrolling unit 225 a to the polarization controller 4B and theinter-polarization-mode delay unit 227, and controls the abovepolarization controller 4B and the inter-polarization-mode delay unit227 such that the above low frequency components included in theintensity of the above first specific frequency component from theintensity detector (first intensity detecting unit) 13 become zero,thereby optimizing a compensation quantity of polarization-modedispersion of the above transmission optical signal.

With the above structure, with respect to a a minute signal at a lowfrequency f₁ (Hz) generated by the low frequency oscillator 235A issuperimposed on an α control signal from the α setting circuit 237A. Apart of the optical signal after polarization-mode dispersioncompensation is split and photoelectrically converted, after that, afrequency fe (Hz) component intensity is extracted so that an intensitydetection is performed. When a value of α is at the optimum positionwhere the frequency fe (Hz) component intensity is the maximum, theextracted frequency fe (Hz) component intensity does not have anintensity changing component of the low frequency f₁ (Hz). When a valueof α is deviated from the optimum position, a component of the frequencyf₁ (Hz) appears in a change with time of the fe (Hz) componentintensity. Accordingly, a component of the frequency f₁ (Hz) detectedfrom the fe (Hz) component intensity is detected, and a feedback isperformed in analog such as to change a value of α in such a directionthat that component disappears.

Namely, the phase comparing circuit 233A compares a phase of thatcomponent with a phase of the low frequency signal f₁ (Hz) from the lowfrequency oscillator 234A, and a direction in which α should be changedis determined according to phase information obtained as a result. Thesimilar control is performed on β and Δτ_(c) as well. However, sincefrequencies of the low frequencies are at different values, it ispossible to independently perform the optimum controls even if thecontrols are performed simultaneously.

Namely, the compensation quantity optimization controlling unit 241modulates an azimuth angle of the ¼ wave plate 4B-11 and an azimuthangle of the ½ wave plate 4B-12 in the polarization controller 4B and adelay quantity between polarization modes of the inter-polarization-modedelay unit 227 with low frequencies at different frequencies, detects anintensity of the first frequency component in a baseband spectrum of atransmission optical signal, and optimizes the azimuth angle of the ¼wave plate 4B-11 and the azimuth angle of the ½ wave plate 4B-12 in theabove polarization controller 4B and the delay quantity betweenpolarization modes of the inter-polarization-mode delay unit 227 suchthat the intensity modulation component of the low frequency componentincluded therein becomes zero.

As above, minute modulation is performed with different low frequenciesf₁, f₂ and f₃ (Hz) on respective α, β and Δτ_(c) so that the fe (Hz)component intensity is automatically fixed to the maximum value, whichenables an accurate control.

(C3) Description of a Third Modification of the Second Embodiment

FIG. 40 is a diagram showing a structure of an optical transmissionsystem according to a third modification of the second embodiment ofthis invention, in which an object of control is changeable betweenbefore system operation (before start of system operation) and duringsystem operation (after start of system operation), and the detectionform 1 and the control mode 2 are performed.

The optical transmission system 210E shown in FIG. 40 is as well anoptical communication system with a transmission rate B (b/s) (forexample, 10 Gb/s or the like) adopting time division multiplexing. Inthe optical transmission system 210E, an optical transmitter 2 and anoptical receiver 207 a are connected over an optical transmission line(transmission fiber) 3, and a PMD compensation controlling unit(polarization-mode dispersion controlling unit) 225 b is disposed on thereceiving side. The optical transmitter 2, the optical receiver 207 aand the optical transmission line 3 are the same as those describedabove, further descriptions of which are thus omitted. Incidentally, inthis modification, a term “dispersion” is used to mean“polarization-mode dispersion”.

The dispersion compensation controlling apparatus 225 b comprises aphoto receiver 11, a band-pass filter 12 and an intensity detector 13along with an α·β·Δτ_(c) setting circuit 226. The photo receiver 11, theband-pass filter 12 and the intensity detector 13 have the samefunctions as those described above, further descriptions of which arethus omitted.

The α·β·Δτ_(c) setting circuit 266 can control a polarization controller4B and an inter-polarization-mode variable delay unit 227. Namely, theα·β·Δτ_(c) setting circuit 226 controls an azimuth angle of a ¼ waveplate 4B-11 and an azimuth angle of a ½ wave plate 4B-12 in thepolarization controller 4B and a delay quantity between polarizationmodes of the inter-polarization-mode delay unit 227 in the opticalreceiver 207 a according to a signal inputted from the intensitydetector 13, and optimizes the delay quantity Δτ_(c) of theinter-polarization-mode variable delay unit 227 in order to be able tocomply with fluctuation information (this information is transmitted bythe optical transmitter 2) on a transmission line polarization-modedispersion quantity such as bit rate, transmission distance, signalmodulation system and the like. In consequence, it is necessary toprovide an inter-polarization-mode variable delay element.

With the above structure, a delay quantity Δτ_(c) of theinter-polarization-mode variable delay element is optimized before startof system operation (before operation), and a process for complying withfluctuations in transmission line polarization-mode dispersion quantitydue to a switching of bit rate, transmission distance, signal modulationsystem or the like is performed. When there is a switching in thesystem, a signal informing of the switching is sent from the opticaltransmitter 2 to the polarization-mode dispersion controlling unit 225b, and the delay quantity Δτ_(c) of the inter-polarization-mode variabledelay element is optimized only immediately after the switching. To thecontrary, during system operation, when a fluctuation due to a change inenvironment of the polarization dispersion quantity is smaller than aPMD tolerance (the maximum allowable polarization-mode dispersionquantity), a control on Δτ_(c) is not performed but only α and β arecontrolled.

Whereby, the polarization-mode dispersion controlling unit (PMDcompensation controlling unit) 225 b performs a control on only thepolarization controller 4B during system operation, while controllingthe inter-polarization-mode delay unit 227 when system operation isstarted or when there is a switching of an element determiningconditions of polarization-mode dispersion in the optical transmissionline 3.

(C4) Description of a Fourth Modification of the Second Embodiment

FIG. 41 is a block diagram showing a structure of an opticaltransmission system to which a polarization-mode dispersion compensationcontrolling apparatus at the time of system operation according to afourth modification of the second embodiment of this invention isapplied. The optical transmission system 210F shown in FIG. 41 is aswell an optical communication system with a transmission rate B (b/s)(for example, 10 Gb/s or the like) adopting time division multiplexing,which differs from the system shown in FIG. 40 in that aninter-polarization-mode variable delay unit 230 uses an element of afixed value Δτ_(c). In this modification, a term “dispersion” is used tomean “polarization-mode dispersion”, as well.

In the optical transmission system 210F, an optical transmitter 2 and anoptical receiver 207 b are connected over an optical transmission line(optical fiber) 3, and a dispersion compensation controlling apparatus225 c is disposed on the receiving side. The optical receiver 207 bcomprises a polarization controller 4B, the inter-polarization-modevariable delay unit 230, an optical splitting unit 5 and an opticalreceiving unit 6. The dispersion compensation controlling apparatus 225c comprises a photo receiver 11, a band-pass filter 12, an intensitydetector 13 and an α·β setting circuit 212. The α·β setting circuit 212controls values of α and β such that the fe (Hz) component intensitybecomes the maximum. In other words, the α·β setting circuit 212functions as a means controlling the polarization controller 4B changinga polarization state of an optical signal.

With the above structure, a PMD tolerance is measured using theinter-polarization-mode variable delay unit 230 before system operation,whereas a delay quantity of the inter-polarization-mode variable delayunit 230 is used while the delay quantity is fixed within an allowablerange based on a value of the PMD tolerance, during system operation.

Measurement of the PMD tolerance before system operation is performedusing the same transmitter and receiver as the ones used in the opticaltransmission system 210F. This method will be described with referenceto FIG. 42.

FIG. 42 is a diagram illustrating a method of measuring a PMD tolerance.Before system operation, a PMD tolerance is measured using the sametransmitter and receiver as those used in actual transmission as theoptical receiver 207 b shown in FIG. 42. In concrete, by continuouslychanging a delay quantity between polarization modes using theinter-polarization-mode variable delay unit 230 shown in FIG. 42, apolarization-mode dispersion quantity Δτ_(F) of a transmission line issimulated, and a bit error rate is measured in the optical receiver 207b. Provided that penalty 1 dB or below is transmittable, for example, aPMD tolerance is determined as Δτ_(1 dB). After that, the dispersioncompensation controlling apparatus 225 c sets the delay quantity Δτ_(c)of the inter-polarization-mode variable delay unit 230 in a range ofT−Δτ_(1 dB)<Δτ_(c)<2Δτ_(1 dB), where T represents one time slot period.A way of setting in such range will be described later.

On the other hand, when the system is actually operated, a delayquantity of the inter-polarization-mode variable delay unit 230 is fixedto a predetermined set value, and inserted in a transmission line, andused, as shown in FIGS. 41 and 42.

Whereby, before system operation, a PMD tolerance is measured inresponse to a control signal. As this, it is advantageous thatcompensation conditions of polarization-mode dispersion are optimized inthe initial stage, and parameters to be controlled at the time of systemoperation is lessen.

Next, that such compensation conditions can be optimized will bedescribed with reference to FIGS. 43 through 51, and a reason why thedelay quantity Δτ_(c) of the inter-polarization-mode variable delay unit230 is set in a range of T−Δτ_(1 dB)<Δτ_(c)<2Δτ_(1 dB) will bedescribed.

First, here is shown, using FIG. 43 and formulae (4) through (9) below,that an optical signal after given a delay difference Δτ_(c) betweenpolarization modes by a PMF 231 for polarization-mode dispersioncompensation is expressed by formula (9).

FIG. 43 is a block diagram showing a structure of an opticaltransmission system to which a polarization-mode dispersion compensationcontrolling apparatus using a PMF for polarization-mode dispersioncompensation according to the fourth modification of the secondembodiment of this invention is applied. The optical transmission system210G is as well an optical communication system with a transmission rateB (b/s) (for example, 10 Gb/s or the like) adopting time divisionmultiplexing, which has a function of compensating polarization-modedispersion of a transmission line. In the optical transmission system210G, an optical transmitter 2 and an optical receiver 207 c areconnected over an optical transmission line (transmission fiber) 3, anda dispersion compensation controlling apparatus 225 d is disposed on thereceiving side. The optical receiver 207 c comprises a polarizationcontroller 4B, a polarization maintaining fiber (PMF) 231, an opticalsplitting unit 5 and an optical receiving unit 6.

With these, in the optical receiver 207 c, a received light passesthrough the polarization controller 4B, and is inputted to the PMF 231.Here, the optical signal is polarization-mode-dispersion-compensated(given a delay difference Δτ_(c) between polarization modes), and splitby the optical splitting unit 5. One of the optical signal undergoes alight receiving process in the optical receiving unit 6. With respect tothe other optical signal, an fe=B/2 (Hz) component intensity in abaseband spectrum of a signal at B (Gb/s) is detected in the band-passfilter 12 of the dispersion compensation controlling apparatus 225 d.

Here, when a transmission light is linearly polarized light expressed bya formula (4) below in Jones vector representation, the transmit lightsplit into polarization mode components at an intensity ratio γ due topolarization-mode dispersion of the transmission line, and given a delaydifference Δτ_(F) is expressed in vector representation as a formula(5): $\begin{matrix}\begin{pmatrix}{{A(t)} + {j \cdot {B(t)}}} \\0\end{pmatrix} & (4)\end{matrix}$

(where j is imaginary unit,) $\begin{matrix}{P = \begin{pmatrix}{\sqrt{\gamma} \cdot \left( {{A\left( {t - {\Delta \quad \tau_{F}}} \right)} + {j \cdot {B\left( {t - {\Delta \quad \tau_{F}}} \right)}}} \right)} \\{\sqrt{\left( {1 - \gamma} \right)} \cdot \left( {{A(t)} + {j \cdot {B(t)}}} \right)}\end{pmatrix}} & (5)\end{matrix}$

Further, when azimuth angles of the ¼ wave plate 4B-11 and the ½ waveplate 4B-12 in the polarization controller 4B are α a and β (radian), anoptical waveform after passing through the polarization controller 4B isdetermined in matrix calculation as a formula (6) with matrixesexpressed by formulae (7) and (8): $\begin{matrix}{R = {H \cdot Q \cdot P}} & (6) \\{Q = {\frac{1}{\sqrt{2}}\quad \begin{pmatrix}{1 + {j \cdot {\cos \left( {2\alpha} \right)}}} & {j \cdot {\sin \left( {2\alpha} \right)}} \\{j \cdot {\sin \left( {2\alpha} \right)}} & {1 - {j \cdot {\cos \left( {2\alpha} \right)}}}\end{pmatrix}}} & (7) \\{H = {j\quad \begin{pmatrix}{\cos \left( {2\beta} \right)} & {\sin \left( {2\beta} \right)} \\{\sin \left( {2\beta} \right)} & {- {\cos \left( {2\beta} \right)}}\end{pmatrix}}} & (8)\end{matrix}$

As a result, assuming that the optical waveform is expressed in a formof R below: $R = \begin{pmatrix}{{C(t)} + {j \cdot {D(t)}}} \\{{E(t)} + {j \cdot {F(t)}}}\end{pmatrix}$

The optical waveform after given a delay difference Δτ_(F) betweenpolarization modes by the PMF 231 for PMD compensation is finallyexpressed by a formula (9): $\begin{matrix}{R^{\prime} = \begin{pmatrix}{{C\left( {t + \Delta_{\tau_{c}}} \right)} + {j \cdot {D\left( {t + {\Delta \quad \tau_{c}}} \right)}}} \\{{E(t)} + {j \cdot {F(t)}}}\end{pmatrix}} & (9)\end{matrix}$

Here, although axes are such set that the fast axis of polarization-modedispersion of the transmission line and the slow axis of the PMF are inparallel for the sake of convenience, it is possible to realize thesimilar state by adjusting α (QWP) and β (HWP) even if they are in arotated relation, in general.

Next, here is shown that α and β yielding the maximum value of the 20GHz component intensity are the same as α and β yielding the maximumvalue of the eye opening, with reference to FIGS. 44 through 47. Thesedrawings are made in computer simulation, wherein the transverse axis isα (QWP), while the vertical axis is β (HWP), and the magnitude of theintensity is represented by contour lines along the Z axis (in adirection penetrating from the back of the paper to the front thereof).

FIGS. 44(a) and 44(b) show 20 GHz component intensity in a receivedbaseband signal and eye opening of a received waveform with respect to α(degree: degree) and β (degree: degree) when these controls areperformed on an NRZ signal where a delay quantity Δτ_(c) is 0 (ps). Fromthese drawings, it is seen that a combination of α and β yielding themaximum of the 20 GHz component intensity [peaks of the contour lines inFIG. 44(a), parts denoted by 1 through 8] and a combination of α and βyielding the maximum of the eye opening [peaks of the contour line,parts denoted by 1 through 8 in FIG. 448b] coincide. Incidentally, γ isfixed to 0.5 so that waveform deterioration due to polarization-modedispersion is the maximum.

Similarly, FIGS. 45(a) and 45(b) show 20 GHz component intensity in areceived baseband signal and eye opening of a received waveform where adelay quantity Δτ_(F) is 5 (ps), wherein positions 1 through 7 of peaksof the contour lines in FIG. 45(a) coincide with positions 1 through 7of peaks of the contour lines in FIG. 45(b). FIGS. 46(a) and 46(b) show20 GHz component intensity of a received baseband signal and eye openingof a received waveform where a delay quantity Δτ_(F) is 10 (ps), whereinpositions 1 through 6 of peaks of the contour lines in FIG. 46(a)coincide with positions 1 through 6 of peaks of the contour lines inFIG. 46(b). Further, FIGS. 47(a) and 47(b) show 20 GHz componentintensity in a received baseband signal and eye opening of a receivedwaveform where a delay quantity Δτ_(F) is 20 (ps), wherein positions 1through 7 of peaks of the contour lines in FIG. 47(a) coincide withpositions 1 through 7 of peaks of the contour lines in FIG. 47(b).

From FIGS. 44(a) and 44(b) through 47(a) and 47(b) all, it is seen thata combination of α and β yielding the maximum and the minimum of the 20GHz component intensity and a combination of α and β yielding themaximum and the minimum at the peaks of the contour lines) of the eyeopening coincide to each other. This relationship coincides with respectto the PMD value (polarization-mode dispersion value of all transmissionlines. From this, it is seen that a polarization-mode dispersioncompensating method maximizing a frequency component intensity that is ahalf of the bit rate in the baseband spectrum is effective. With respectto not only the NRZ signal but also the 40 Gb/s OTDM waveform, it hasbeen confirmed that a control using the 20 GHz component intensity canbe performed in the similar manner.

Next, eye opening penalty will be described with reference to FIGS.48(a) and 48(b), and 49(a) and 49(b).

FIG. 48(a) is a diagram showing results of calculation of transmissionpath PMD versus 20 GHz component intensity in the case wheretransmission is performed using a 40 Gb/s NRZ signal with/withoutpolarization-mode dispersion compensation, wherein two kinds, withcompensation and without compensation, are shown. The transverse axis inFIG. 48(a) shows PMD Δτ_(F) of the transmissin line, whereas thevertical axis shows the maximum value of 20 GHz component intensity.Δτ_(F) shows a delay quantity of the transmission path, and eye openingpenalty signifies an amount of the eye opening deteriorated from whenthe transmitter and the receiver face to each other. FIG. 48(b) is adiagram showing results of calculation of transmission PMD Δτ_(F)(transverse axis) versus eye opening penalty (vertical axis). Here,Δτ_(F)=10 ps and 20 ps are set as a value of PMF for compensation.Incidentally, values of α and β are varied according to a value ofΔτ_(F), and these values are such set that they become the optimumvalues when they are calculated.

As shown in FIGS. 48(a) and 48(b), in the case of Δτ_(F)=10 ps, acombination of α=45° and β=22.5° yields the maximum 20 GHz componentintensity (refer to FIG. 48(a)), and yeilds the minimum value 0 of theeye opening penalty (refer to FIG. 48(b)) when the transmission PMD isΔτ_(F)=0 ps. This corresponds to a case where an optical signal inputtedto the PMF 231 is linearly polarized light coinciding with thepolarization primary axis direction of the PMF 231 by the polarizationcontroller 4B. In this case, the optical signal is not affected bypolarization dispersion of the PMF 231 so that the same eye opening isobtained as when the transmitter and the receiver face to each other.

To the contrary, if the transmission line PMD is as sufficiently largeas Δτ_(F)>10 ps, the 20 GHz component intensity is the maximum whenα=β=0 [refer to FIG. 49(a)], and the eye opening penalty is the minimum.This corresponds to a case where a polarization direction of lightpassing through the fast axis of the polarization primary axis of thetransmission line coincides with the slow axis of the PMF 23, while apolarization direction of light passing through the slow axis coincideswith the fast axis of the PMF. As a result, this coincides with a statewhere the optical waveform is subjected to polarization dispersion of adeduction of Δτ_(F)−Δτ_(c), and deterioration is therefore moresuppressed than in the a case where the optical waveform is subjected topolarization dispersion of Δτ_(F) without compensation.

In the case of an intermediate range of 0 ps<Δτ_(F)<10 ps, a combinationof α and β yielding the maximum 20 GHz component intensity continuouslychanges from α=45° and β=22.5° to α=β=0° with increasing Δτ_(F). In suchcase, the eye opening penalty increases from 0 dB at a point in thevicinity of Δτ_(F)Δτ_(c)/2=5 ps, after that, slightly decreases, andagain becomes 0 db at Δτ_(F)=Δτ_(c)=10 ps. Back to FIG. 48(b), when thePMF for compensation is Δτ_(c)=20 ps, it is seen that an increase of thepanelty in the vicinity of Δτ_(F)=Δτ_(c)/2=10 ps is noticeable. In orderto compensate polarization-mode dispersion within as a larger rangeΔτ_(F) as possible, it is necessary to set Δτ_(c) to a large value tosome extent. But, if Δτ_(F) is set to an excessively lager value, anincrease of the penalty at Δτ_(F)=Δτ_(c)/2 becomes large. Therefore,there exists a range of appropriate Δτ_(c).

Similarly, FIGS. 49(a) and 49(b) show results of calculation oftransmission path PMD Δτ_(F) versus 20 GHz component intensity, andtransmission path PMD Δτ_(F) versus eye opening penalty whentransmission is performed with a 40 Gb/s OTDM signal with/withoutpolarization-mode dispersion compensation, wherein two kinds, that is,with compensation and without compensation, are shown. In this case,results similar to those in the case of an NRZ signal are obtained.

Next, a way of determining a set range of appropriate Δτ will bedescribed with reference to FIGS. 50(a) and 50(b). FIG. 50(a) is adiagram showing a relationship of transmission path PMD versus eyeopening penalty in the case where the delay quantity Δτ_(c) is theminimum. FIG. 50(b) is a diagram showing a relationship of transmissionPMD versus eye opening penalty in the case where the delay quantityΔτ_(c) is the maximum. These drawings are drawings schematically showinga way of determining the set range, in which what shown by a broken linea and a broken line b are penalty changes (relationship of PMD Δτ_(F)versus penalty) due to polarization-mode dispersion without apolarization-mode dispersion compensator, and what shown by a solid lineare penalty changes due to polarization-mode dispersion at the time ofpolarization-mode dispersion with PMF. Here, when penalty 1 db or belowis a deterioration allowable reference, a PMD tolerance (the maximumallowable polarization-mode dispersion quantity) is indicated at 1 dBalong Δτ (vertical axis) in FIGS. 50(a) and 50(b). Incidentally,Δτ_(max) along the transverse axis corresponds to a period of one timeslot.

In Δ_(τ) _(F)>Δτ_(c) in a part indicated by a solid line in FIG. 50(a)(relationship of transmission line PMD Δτ_(F) versus penalty whenpolarization-mode dispersion compensation is performed with thepolarization controller and the PMF with a delay difference Δτ_(c)), thepenalty changes follow a solid line c that is obtained by moving abroken line a in parallel by Δτ in a direction of the Δτ_(F) axis. In0<Δτ_(F)<Δτ_(c), there is a penalty increase with the maximum at a pointB in the vicinity of Δτ_(F)=Δτ_(c)/2, but the penalty thereat is smallerthan that at an intersection of a broken line d and a broken line a thatis obtained by moving the broken line b by Δτ_(c) in a direction of theΔτ_(F) axis in parallel. When Δτ_(c) is set to a large value, a point Bmoves in a direction closer to a point A, and exceeds the deteriorationallowable reference of below 1 dB. Therefore, it is necessary that thepoint A satisfies the allowable reference of penalty 1 dB or below. Inconsequence, as shown in FIG. 50(b), the maximum value of Δτ_(c) is adouble of a PMD tolerance Δτ_(1 dB), and the point A is practically apoint reaching the deterioration allowable reference.

When Δτ_(F) exceeds one time slot, this polarization-mode dispersioncompensating method cannot be principally applied. A reason of this willbe described with reference to FIGS. 51(a) and 51(b). FIGS. 51(a) and51(b) are diagrams illustrating a case where a delay quantity Δ96exceeds one time slot. When Δτ_(F) exceeds one time slot (25 ps) asshown in FIG. 51(b), the monitor intensity becomes the maximum whenΔτ_(T)=Δτ_(F)+Δτ_(c). Namely, in FIG. 51(b), although waveformdeterioration in the case of a combination of α and β by which a totalPMD quantity after polarization-mode dispersion compensation isΔτ_(T)=Δτ_(F)−Δτ_(c) is larger than that in the case where a combinationof α and β by which Δτ_(T)=Δτ_(F)−Δτ_(c), the detected B/2 GHz componentintensity is larger.

When Δτ_(F)=(one time slot) is assumed to be the maximum value of thepolarization-mode dispersion quantity in polarization-mode dispersioncompensation, Δτ_(c)=(one time slot)−(PMD tolerance Δτ_(1 dB)), as shownin FIG. 50(b).

Namely, the polarization-mode dispersion controlling unit 225 c (referto FIG. 41) comprises a maximum allowable polarization-mode dispersionsetting means (α·β setting circuit 212) setting a maximum allowablepolarization-mode dispersion quantity. In addition, when thepolarization-mode dispersion controlling unit 225C feedback-controls atleast either the polarization controller 4B or theinter-polarization-mode delay unit 230 disposed in the opticaltransmission line 3 such that an intensity of the frequency componentcorresponding to ½ of the bit rate as the first specific frequencycomponent detected by the first intensity detecting unit (intensitydetector 13) becomes the maximum, the polarization-mode dispersioncontrolling unit 225C sets a delay quantity Δτ_(c) of theinter-polarization-mode delay unit 230 to a value above the lower limitvalue defined as a value obtained by subtracting the maximum allowablepolarization-mode dispersion quantity Δτ_(1 dB) from one time slot andbelow the upper limit value defined as a value having a magnitude twotimes the maximum allowable polarization-mode dispersion quantityΔτ_(1 dB), at the time of system operation. The polarization-modedispersion controlling unit 225 c may set a delay quantity of theinter-polarization-mode delay unit 230 at the time of system operationto the upper limit value or the lower limit value.

Again back to FIG. 41, the dispersion compensation controlling apparatus225 c measures a PMD tolerance using the same transceiver as one used inan actual optical transmission system before system operation, anddetermines a PMD tolerance Δτ_(1 dB) with penalty 1 dB as a reference oftransmission capability. After that, the dispersion compensationcontrolling apparatus 225 c sets a delay quantity Δτ_(c) of theinter-polarization-mode variable delay unit 230 to a range ofT−Δτ_(1 dB)<Δτ_(c)<2Δτ_(1 dB).

As above, it is advantageously possible to optimize compensationconditions of polarization-mode dispersion in the initial stage, andlessen parameters to be controlled at the time of system operation. Ifthe control mode is developed to a control mode in which aninter-polarization-mode variable delay element (refer to FIG. 41) isused to control in lieu of an inter-polarization-mode fixed delayelement as the PMF 231 (refer to FIG. 43), it is possible to more lessenwaveform deterioration. Namely, by performing a control such as toharmonize a delay quantity Δτ_(c) of the variable delay element with aPMD quantity Δτ_(F) of the transmission line, it is possible to make aPMD quantity after compensation be Δτ_(T)=Δτ_(F)−Δτ_(c)=0. As above, itis possible to effectively perform polarization-mode dispersioncompensation of a transmission line that is a transmission limitingfactor in a very high-speed optical transmission system.

(D) Description of a Third Embodiment

In actual transmission, both of chromatic dispersion andpolarization-mode dispersion of a transmission line become factorslimiting a transmission rate and a transmission distance. In order toovercome them, it is required a system simultaneously monitoring achromatic dispersion value and a polarization-mode dispersion value ofthe transmission line, and simultaneously compensating transmissionoptical waveform deterioration due to them. Although a term “dispersion”is generally used to mean “chromatic dispersion”, the term “dispersion”is used to mean both “polarization-mode dispersion” and “chromaticdispersion” in a third embodiment.

An optical transmission system 70 shown in FIG. 52 is an opticalcommunication system with a transmission rate B (b/s) (for example, 40Gb/s, 10 Gb/s or the like) adopting time division multiplexing. Theoptical transmission system 70 differs from the optical transmissionsystem 10 according to the first embodiment in that the opticaltransmission system 70 compensates not only polarization-mode dispersionof a transmission optical signal but also chromatic dispersion of thetransmission optical signal, the other parts of which are similar tothose of the optical transmission system 10 according to the firstembodiment.

Namely, in the optical transmission system 70, an optical transmitter 72as a transmitting terminal apparatus transmitting a transmission opticalsignal and an optical receiver 77 as a receiving terminal apparatusreceiving the transmission optical signal are connected over an opticaltransmission line (transmission fiber) 73, and a dispersion compensationcontrolling apparatus 71 is disposed on the receiving side.Incidentally, the dispersion compensation controlling apparatus 71signifies “polarization-mode dispersion-chromatic dispersioncompensation controlling apparatus 71”.

The optical receiver 77 comprises a chromatic dispersion compensator 83,a polarization-mode dispersion compensator 74, an optical splitting unitand an optical receiving unit 76. The chromatic dispersion compensator83 compensates chromatic dispersion of a transmission optical signal.The polarization-mode dispersion compensator 74 compensatespolarization-mode dispersion generated in a transmitted optical signal.Incidentally, the optical splitting unit 74 and the optical receivingunit 76 are similar to those described above, further descriptions ofwhich are thus omitted.

The dispersion compensation controlling apparatus 71 monitors a state ofpolarization-mode dispersion and a state of chromatic dispersiongenerated in an optical signal transmitted over the optical transmissionline 73 on the basis of an optical signal taken out by the opticalsplitting unit 75, and controls the polarization-mode dispersioncompensator 74 and the chromatic dispersion compensator 83 according toresults of the monitoring, which comprises a photo receiver 78, aband-pass filter [B/2 (Hz) BPF] 79A, a band-pass filter [B (Hz) BPF]79B, intensity detectors 80A and 80B, a polarization-mode dispersioncontrolling unit 91 and a chromatic dispersion controlling unit 240.

The photo receiver 78 receives an optical signal taken out by theoptical splitting unit 75, and converts it into an electric signal. Theband-pass filter [B/2 (Hz) BPF] 79A detects a first specific frequencycomponent [B/s (Hz) component] in a baseband spectrum in a transmissionoptical signal inputted to the receiving side over the opticaltransmission line 73, which functions as a first specific frequencycomponent detecting unit. The first specific frequency component isappropriately set according to a transmission rate or a signal waveformof an optical signal, a frequency of which is set to a frequencycorresponding to ½ of the bit rate.

The intensity detector 80A detects information on an intensity of theabove first specific frequency component detected by the band-passfilter 79A, which functions as a first intensity detecting unit.

The polarization-mode dispersion controlling unit 91 controls apolarization-mode dispersion quantity of the optical transmission line73 such that the intensity of the first specific frequency componentdetected by the intensity detector 80A is the maximum, which comprises apolarization-mode dispersion quantity detecting unit 81 and a chromaticdispersion quantity detecting unit 81B. The polarization-mode dispersionquantity detecting unit 81 detects a polarization-mode dispersionquantity. A parameter setting circuit 82 outputs a parameter settingcontrol signal having parameter information as a control quantity forcompensating polarization-mode dispersion of the transmission opticalsignal on the basis of the polarization-mode dispersion quantitydetected by the polarization-mode dispersion quantity detecting unit 81to the polarization-mode dispersion compensator 74 in the opticalreceiver 77. The polarization-mode dispersion controlling unit 91 uses acontrol mode 2 (or a control mode 12 to be described later).

The band-pass filter [B (Hz) BPF] 79B detects a second specificfrequency component [B (Hz) component] in a baseband spectrum in atransmission optical signal inputted to the receiving side over theoptical transmission line 73, which functions as a second specificfrequency component detecting unit. A frequency of the second specificfrequency component is set to a frequency corresponding to the bit rate.The intensity detector 80B detects information on an intensity fo theabove second specific frequency component detected by the band-passfilter 79B, which functions as a second intensity detecting unit.Incidentally, the intensity detector 80B may output information on theintensity of the above second specific frequency component detected bythe intensity detector 80B as a monitor signal.

The chromatic dispersion controlling unit 240 controls a chromaticdispersion quantity of the transmission line 73 such that the intensityof the second specific frequency component detected by the intensitydetector 80B becomes the maximum or the minimum, which comprises thechromatic dispersion quantity detecting unit 81B and a chromaticdispersion quantity setting circuit 82B.

The chromatic dispersion quantity detecting unit 81B detects a chromaticdispersion quantity of the above transmission optical signal from theintensity of the above second specific frequency component detected bythe intensity detector 80B by performing a predetermined secondfunctional operation. The chromatic dispersion compensation quantitysetting circuit 82B sets a chromatic dispersion control quantity in thechromatic dispersion compensator 83 disposed in the optical transmissionline 73 in order to compensate chromatic dispersion of the abovetransmission optical signal on the basis of the above chromaticdispersion quantity detected by the chromatic dispersion quantitydetecting unit 81B, which functions as a chromatic dispersion controlquantity setting unit.

This control method is a method in which the chromatic dispersioncompensator 83 disposed in the optical transmission line 73 isfeedback-controlled such that the intensity of the second specificfrequency component detected by the second intensity detector (intensitydetecting unit 80B) becomes the maximum or the minimum. Namely, acontrol quantity is determined by feeding-back such that the intensityof the detected specific frequency becomes the maximum or the minimumwithout the first function. Although the control mode 2 is defined as“becoming the maximum” in the above second embodiment, the control mode2 will include a feedback control such as “becoming the maximum or theminimum” hereinafter.

In simultaneous monitoring of a chromatic dispersion value and apolarization-mode dispersion value, the same frequency B (Hz) as thetransmission rate B (b/s) is used as a chromatic dispersion monitorfrequency F_(GVD), while a frequency B/2 (Hz) that is a half of thetransmission rate B (b/s) is used as a PMD monitor frequency f_(PMD),which are different from each other. According to this embodiment, anoutput of the photo receiver 78 is split into two, and frequency valuesat which the specific frequency components are detected are of twokinds, but the detection form 1 is used. A reason why the detection form2 is not used is that a signal system (the band-pass filter 79A, theintensity detector 80A and the polarization-mode dispersion controllingunit 91) performing a polarization-mode dispersion control uses one kindof frequency, while a signal system (the band-pass filter 79B, theintensity detector 80B and the chromatic dispersion controlling unit240) performing a chromatic dispersion control also uses one kind offrequency.

According to this embodiment, when the above transmission optical signalis an NRZ optical signal, the first specific frequency componentdetecting unit (the band-pass filter 79A) detects a frequencycorresponding to a half of the bit rate as the first specific frequencycomponent, while the second specific frequency component detecting unit(the band-pass filter 79B) detects a frequency corresponding to the bitrate as the second specific frequency component. In the case of the 40Gb/s NRZ system, there are set f_(GVD)=40 GHz and f_(PMD)=20 GHz.Incidentally, as an example of frequency setting, a value other than theabove may be used.

From this, even if polarization-mode dispersion and chromatic dispersionhave dependency on each other, it is possible to perform the controlssimultaneously and independently. For example, in the case of the 40Gb/s NRZ system, it is sufficient to control such that the F_(GDV)=40GHz intensity becomes the minimum, while the f_(PMD)=20 GHz intensitybecomes the maximum.

Whereby, a flow of an otpical signal is as follows. An optical signal ata transmission rate B (b/s) transmitted from the optical transmitter 72is transmitted to the optical receiver 77 over the optical transmissionline 73, a part of the optical signal transmitted over the opticaltransmission line 73 is taken out by the optical splitting unit 75, andthe optical signal (monitor light) taken out is sent to the dispersionquantity detecting apparatus 71.

In the dispersion quantity detecting apparatus 71, a state ofpolarization-mode dispersion and a state of chromatic dispersiongenerated in the optical signal transmitted over the opticaltransmission line 73 are monitored on the basis of the optical signaltaken out by the optical splitting unit 75, and a control in the controlmode 2 is performed by the polarization-mode dispersion compensator 74and the chromatic dispersion compensator 83 according to results of themonitoring. Namely, a maximum value control is such performed thatpredetermined compensation values are obtained.

In concrete, the optical signal taken out by the optical splitter 75 isreceived by the photo receiver 78, converted into an electric signal,and inputted to the band-pass filters 79A and 79B. In the band-passfilter 79A, the first specific frequency component [B/2 (Hz) component]in a baseband spectrum in the transmission optical signal is detected,an intensity of the above first specific frequency component detected bythe band-pass filter 79A is detected by the intensity detector 80A, andthe feedback control is performed such that the intensity of thespecific frequency component becomes the maximum or the minimum.

From the parameter setting circuit 82, a parameter setting signal forsetting such parameter information (delay quantity Δτ) as to cancel apolarization-mode dispersion quantity detected by the polarization-modedispersion quantity detecting unit 81 to the polarization-modedispersion compensator 74 disposed in the optical receiver 77 in orderto compensate polarization-mode dispersion in the transmission opticalsignal.

In the polarization-mode dispersion compensator 74, parameterinformation is set on the basis of the control signal when thepolarization-mode dispersion compensator 74 receives the parametersetting control signal, polarization-mode dispersion generated in theoptical signal transmitted over the optical line 74 is therebycompensated.

On the other hand, in the band-pass filter 79B in the dispersioncompensation controlling apparatus 71, the second specific frequencycomponent [B (Hz) component] in the baseband spectrum in thetransmission optical signal is detected, an intensity of the abovesecond specific frequency component detected by the band-pass filter 79Bis detected by the intensity detector 80B, and a feedback control issuch performed that the intensity of the specific frequency componentbecomes the maximum or the minimum.

In the chromatic dispersion compensator 83, chromatic dispersiongenerated in the optical signal transmitted over the opticaltransmission line 73 is compensated on the basis of the control signalwhen the chromatic dispersion compensator 83 receives the controlsignal. Namely, this dispersion compensation controlling steps are asfollows. The first specific frequency component in a baseband spectrumin a transmission optical signal inputted to the receiving side over atransmission fiber as the transmission line is detected (first specificfrequency component detecting step), information on an intensity of theabove first specific frequency component detected at the first specificfrequency component detecting step is detected (first intensitydetecting step), a polarization-mode dispersion quantity of the opticaltransmission line 73 is such controlled that the intensity of the firstspecific frequency component detected at the first intensity detectingstep becomes the maximum (polarization-mode dispersion controllingstep), the second specific frequency component in the baseband spectrumin the transmission optical signal is detected (second specificfrequency component detecting step), information on an intensity of theabove second specific frequency component detected at the secondspecific frequency component detecting step is detected (secondintensity detecting step), and a chromatic dispersion quantity of theoptical transmission line 73 is such controlled that the intensity ofthe second specific frequency component detected at the second intensitydetecting step becomes the maximum or the minimum (chromatic dispersioncontrolling step).

Whereby, the controls can be performed independently and simultaneously.

With the above structure, it is possible to simultaneously optimizechromatic dispersion compensation and polarization-mode dispersioncompensation that become factors limiting a transmission rate and atransmission distance in a very high-speed optical transmission systemin TDM system.

As above, according to the dispersion compensation controlling apparatus71 of the third modification of this invention, it is possible to attainthe similar advantages to the first embodiment described above. Since itis also possible to compensate not only polarization-mode dispersion ofa transmission optical signal but also chromatic dispersion of thetransmission optical signal, this embodiment can prevent deteriorationof a transmission waveform of an optical signal due to effects bypolarization-mode dispersion and chromatic dispersion, and furthercontributes to long-distance transmission of a high-speed opticalsignal.

Conversely, it is possible to perform a control in the control mode 1 ona polarization-mode dispersion control quantity and a chromaticdispersion control quantity. Namely, a polarization-mode dispersionquantity can be determined with the first function, and a chromaticdispersion control quantity can be determined with the second function.Here, determining with the first function means that a polarization-modedispersion quantity of the above transmission optical signal is detectedby performing a predetermined first functional operation [that is, afunctional operation using the above formulae (2) and (3)]. Determiningwith the second function means that chromatic dispersion valuedependency of a predetermined frequency component intensity is measuredin advance and stored as data, a function based on this data is made,and determined as a second function.

Namely, the polarization-mode dispersion controlling unit(polarization-mode dispersion quantity detecting unit 81, parametersetting circuit 82) may set a polarization-mode dispersion controlquantity in the polarization-mode dispersion compensator 74 disposed inthe optical transmission line 73 such that the intensity of the firstspecific frequency component detected by the first intensity detectingunit becomes the maximum, and the chromatic dispersion controlling unit240 may set a chromatic dispersion control quantity in the chromaticdispersion compensator 83 disposed in the optical transmission line 73such that the intensity of the second specific frequency componentdetected by the intensity detector 80B becomes the maximum or theminimum. Here, the chromatic dispersion quantity detecting unit 81Bdetects a chromatic dispersion quantity of a transmission optical signalfrom the intensity of the above second specific frequency componentdetected by the intensity detector 80B (second intensity detecting unit)by performing an operation with a predetermined second function (secondfunctional operation). The chromatic dispersion compensation quantitysetting circuit 82B sets a chromatic dispersion control quantity in thechromatic dispersion compensator 83 disposed in the optical transmissionline 73 in order to compensate chromatic dispersion in the transmissionoptical signal on the basis of the chromatic dispersion quantitydetected by the chromatic dispersion quantity detecting unit 81B, whichfunctions as a chromatic dispersion quantity setting unit.

A flow of a signal in this case is as follows. Namely, in thepolarization-mode dispersion quantity detecting unit 81 shown in FIG.52, a polarization-mode dispersion quantity of the above transmissionoptical signal is detected from the intensity of the first specificfrequency component detected by the intensity detector 80A by performinga predetermined first functional operation [that is, a functionaloperation using the above formulae (2) and (3)]. In the chromaticdispersion quantity detecting unit 81B, a chromatic dispersion quantityof the above transmission optical signal is detected from the intensityof the second specific frequency component detected by the intensitydetector 80B by performing a predetermined second functional operation.In the chromatic dispersion compensation quantity setting circuit 82B, acontrol signal for setting a chromatic dispersion control quantity isoutputted to the chromatic dispersion compensator 83 disposed in theoptical transmission line 73 on the basis of the above chromaticdispersion quantity detected by the chromatic dispersion quantitydetecting unit 81B in order to compensate chromatic dispersion of theabove transmission optical signal.

As having been described in the above second embodiment, when a feedbackcontrol is automatically performed using low-frequency-superimposing, itis possible to independently control the polarization-mode dispersioncompensator 74 and the chromatic dispersion compensator 83 by usingdifferent frequencies of low frequency signals to be superimposed oncontrol signals for the polarization-mode dispersion compensator 74 andthe chromatic dispersion compensator 83.

FIG. 53 is a block diagram showing a structure of an opticaltransmission system according to the third embodiment of this invention.FIG. 53 is a diagram showing a system equivalent to the opticaltransmission system shown in FIG. 52, which is drawn paying attention toa position at which a signal provided for monitoring is taken out(extracted) when a chromatic dispersion value and a polarization-modedispersion value are simultaneously monitored. Received signal light issplit into two in an optical stage (optical splitting unit 75). One ofthe signal light is inputted as a main signal system to the opticalreceiving unit 76, O/E-converted by a photo receiver 76 a [denoted as PD(Photo Diode) in FIG. 53] and undergoes a receiving process in anoptical receiving unit 76 b (denoted as Rx in FIG. 53). The other isinputted as a monitor system to a photo receiver 78 (denoted as PD inFIG. 53) and O/E-converted, and an electric signal is processed.

Further, the light is split into two in an electric stage (photoreceiver 78) and inputted to a narrow band band-pass filter 79B of acenter wavelength F_(GVD) (Hz), a monitor value is detected by anintensity detector 80A, while the other is inputted to a narrow-bandband-pass filter 79B of a center wavelength f_(PMD), and a monitor valueis detected by an intensity detector 80B. Namely, a first intensitydetecting unit (intensity detector 80A) can output information on theintensity of the above first specific frequency component as a monitorsignal, while a second intensity detecting unit (intensity detector 80B)can output information on the detected intensity of the above secondspecific frequency component as a monitor signal. Incidentally, each ofthe monitoring system uses one kind of frequency value, so that thedetection form 1 is used.

FIG. 54 is a detailed block diagram of an optical transmission systemaccording to the third embodiment of this invention. The opticaltransmission system 70 shown in FIG. 54 comprises an optical transmitter72, an optical transmission line 73, an optical receiver 77 and adispersion compensation controlling apparatus 71.

A chromatic dispersion compensator 83 and a polarization-mode dispersioncompensator 74 in the optical receiver 77 are of variable type, in whicha chromatic dispersion compensation quantity and a polarization-modedispersion compensation quantity can be optimum-value-controlled at anytime during system operation. An output signal from an optical splittingunit 75 is inputted to a photo receiver 78 in the dispersioncompensation controlling apparatus 71. An output of the photo receiver78 is split and inputted to band-pass filters 79A and 79B. Outputs ofthe band-pass filters 79A and 79B are inputted to intensity detectors80A and 80B. Further, outputs of the intensity detectors 80A and 80B areinputted to CPUs 239A and 239B. The CPUs 239A and 239B feedback-controlthe polarization-mode dispersion compensator 74 and the chromaticdispersion compensator 83 arranged in the receiving terminal using asimultaneous monitoring method, which function as a polarization-modedispersion controlling unit and a chromatic dispersion controlling unit.

Incidentally, when a chromatic dispersion quantity and apolarization-mode dispersion quantity are set to optimum values only atthe time of start of system operation, the compensators are notnecessarily “variable”. For example, a “fixed” dispersion compensatorsuch as a dispersion compensating fiber, a dispersion compensator of afiber grating type or the like may be inserted.

As a method of switching the controls when a chromatic dispersioncompensation quantity and a polarization-mode dispersion compensationquantity are optimum-value-controlled at all times during systemoperation, a method in which the above controls are performedindependently and in parallel with respect to time, that is, a method inwhich the above polarization-mode dispersion controlling step and thechromatic dispersion controlling step are executed independently, may beemployed. Or, a method in which the above steps are executed in timeseries in order to prevent them from being overlapped with respect totime, that is, a method in which the above polarization-mode dispersioncontrolling step and the chromatic dispersion controlling step areexectued in time series, may be employed.

Although the polarization-mode dispersion compensator and the chromaticdispersion compensator are controlled by the CPUs 239A and 239B, it isalternatively possible to use a control method by an analog circuitusing synchronous detection or the like, not limited to the aboveexample. It is also possible to insert an A/D converter (not shown) anda D/A converter (not shown) in front of and behind the CPUs 239A and239B.

(D1) Description of a First Modification of the Third Embodiment

As to simultaneous monitoring of a chromatic dispersion value and apolarization-mode dispersion value, a position at which signals to beprovided for the monitoring may be set in a various position to executethe monitoring. FIG. 55 is a diagram showing a block diagram of anoptical transmission system according a first modification of the thirdembodiment of this invention. In the optical transmission system 70Ashown in FIG. 55, an optical transmitter 72 and an optical receiver 77Aare connected over an optical transmission line 73, and a compensationquantity monitoring apparatus 92A is disposed on the receiving side. Thecompensation quantity monitoring apparatus 92A comprises photo receivers(PD) 78A and 78B, band-pass filters 79B and 79A, and intensity detectors80A and 80B. Incidentally, as an example of a frequency setting,f_(GVD)=B (GHz) and f_(PMD)=B/2 (GHz) in the case of a B (Gb/s) NRZsignal. However, values other than the above may be used.

At the receiving terminal, signal light is split into three by anoptical splitting unit 75A (optical stage), one of the split signallight is used in a main signal system (optical receiving unit) and theother two are used for monitoring chromatic dispersion andpolarization-mode dispersion. Further, the monitoring system opticalsignals are received by the optical receivers 78A and 78B, differentfrequency components are extracted by the narrow-band band-pass filters79B and 79A having different center wavelengths f_(GVD) and f_(PMD), andmonitor values are detected by the intensity detectors 80B and 80A.Incidentally, each of the monitoring system uses one kind of frequencyvalue, so that the detection form 1 is used.

FIG. 56 is a block diagram of an optical transmission system accordingto the first modification of the third embodiment of this invention,which shows a structure in the case where attention is further paid to aloop simultaneously compensating chromatic dispersion andpolarization-mode dispersion in FIG. 55. In FIG. 56, the same referencecharacters designate like or corresponding parts in FIG. 55. Here, achromatic dispersion compensator 83 and a polarization-mode compensator74 are of variable type, in which a chromatic dispersion compensationquantity and a polarization-mode dispersion compensation quantity can beoptimum-value-controlled at all times during system operation. In thismodification, a term “dispersion” is used to mean both“polarization-mode dispersion” and “chromatic dispersion”. In the caseof NRZ system, it is only necessary to control such that the f_(GVD)=40(GHz) intensity is the maximum, while the F_(PMD)=20 (GHz) intensity isthe minimum. Whereby, it is possible to perform the controlssimultaneously and independently even if they have dependency on eachother.

Outputs of intensity detectors 80A and 80B shown in FIG. 56 are inputtedto CPUs 239A and 239B (detection form 1). These CPUs 239A and 239B usethe simultaneous monitoring method in FIG. 55, and function as apolarization-mode dispersion controlling unit and a chromatic dispersioncontrolling unit to feedback-control the chromatic dispersioncompensator 83 and the polarization-mode dispersion compensator 74arranged in the receiving terminal.

When a chromatic dispersion compensation quantity and apolarization-mode dispersion compensation quantity are set to theoptimum values only at the time of start of system operation, thecompensators are not necessarily “variable”. For example, a “fixed”dispersion compensator such as a dispersion compensating fiber, adispersion compensator of a fiber grating type or the like may beinserted.

The method of controlling a polarization-mode dispersion quantity and achromatic dispersion quantity may use the control mode 1 using the firstfunction and the second function. As a method of switching the controlswhen a chromatic dispersion compensation quantity and apolarization-mode dispersion compensation quantity areoptimum-value-controlled at all times during system operation, a methodin which the above controls are executed independently and in parallelwith respect to time, that is, a method in which the abovepolarization-mode dispersion controlling step and the chromaticdispersion controlling step are executed independently, may be employed.Or a method in which the above steps are executed in time series inorder to prevent them from being overlapped with respect to time, thatis, a method in which the above polarization-mode dispersion controllingstep and the chromatic dispersion controlling step are executed in timeseries, may be employed.

Further, although the polarization-mode dispersion compensator and thechromatic dispersion compensator are controlled by the CPUs 239A and239B, it is alternatively possible to use a controlling method by ananalog circuit using simultaneous detection or the like, not limited tothe above example. It is also possible to insert an A/D converter (notshown) and a D/A converter (not shown) in front of and behind each ofthe CPUs 239A and 239B.

Incidentally, either one of two different frequency components extractedin the electric stage, or the both may be used for extracting a timingin the main signal system.

(D2) Description of a Second Modification of the Third Embodiment

The signal splitting may be performed in the electric stage. FIG. 57 isa block diagram of an optical transmission system 70B according to asecond modification of the third embodiment of this invention. In theoptical transmission system 70B shown in FIG. 57, an optical transmitter72 and an optical receiver 77B are connected over an opticaltransmission line 73, and a compensation quantity monitoring apparatus92B is disposed on the receiving side. The optical receiver 77Bcomprises a photo receiver 78C, while the compensation quantitymonitoring apparatus 92B comprises band-pass filters 79A and 79B, andintensity detectors 80A and 80B. As an example of setting frequencies,f_(GVD)=B (GHz) and f_(PMD)=B/2 (GHz) in the case of a B (Gb/s) NRZsignal. Values other than the above may be employed.

At the receiving terminal, signal light is received by the photoreceiver 78C in the optical receiver 77B, and split into three in theelectric stage. One of the signal light is inputted to an opticalreceiving unit 76 b as a main signal system, and the other two are usedfor monitoring chromatic dispersion and polarization-mode dispersion.The monitoring system optical signals are received by photo receivers78A and 78B, different frequency components are extracted by narrow-bandband-pass filters 79B and 79A having different center wavelengthsf_(GVD) and f_(PMD) in the electric stage, and monitor values aredetected by the intensity detectors 80B and 80A. Therefore, thedetection form 1 is employed.

FIG. 58 is a block diagram of an optical transmission system accordingto the second modification of the third embodiment of this invention,which shows a structure when paying attention to a loop simultaneouslycompensating chromatic dispersion and polarization-mode dispersion. Achromatic dispersion compensator 83 and a polarization-mode dispersioncompensator 74 are of variable type, in which a chromatic dispersionquantity and a polarization-mode dispersion quantity can beoptimum-value-controlled at all times during system operation. Likereference characters in FIG. 58 designate like or corresponding parts inFIG. 57. In this modification, a term “dispersion” is used to mean both“polarization-mode dispersion” and “chromatic dispersion”.

Outputs of intensity detectors 80A and 80B shown in FIG. 58 are inputtedto CPUs 239A and 239B. These CPUs 239A and 239B use the simultaneousmonitoring method in FIG. 57, function as a polarization-mode dispersioncontrolling unit and a chromatic dispersion controlling unit tofeedback-control the chromatic dispersion compensator 83 and thepolarization-mode dispersion compensator 74 arranged in the receivingterminal.

Incidentally, when a chromatic dispersion compensation quantity and apolarization-mode dispersion compensation quantity are set to theoptimum values only at the time of start of system operation, thecompensators are not necessarily “variable”. For example, a “fixed”dispersion compensator such as a dispersion compensating fiber, adispersion compensator of a fiber grating type or the like may beinserted.

Since it is only necessary to control the compensation values to be themaximum or the minimum values in the control mode 2 in the feedbackcontrol of the compensators, the controls are executed independently andin paralle even if they have dependency on each other. For example, inthe case of 40 Gb/s NRZ system, it is only necessary to control thef_(GVD)=40 GHz intensity to be the minimum while the f_(PMD)=20 GHzintensity to be the maximum. It is alternatively possible to control amonitor value to be an absolute value using the control mode 1.

As a method of switching controls when a chromatic dispersioncompensation value and a polarization-mode dispersion compensation valueare controlled to be the optimum values at all times during systemoperation, a method in which the above controls are executedindependently and in parallel with respect to time, that is, a method inwhich the above polarization-mode dispersion controlling step and thechromatic dispersion controlling step are executed independently, may beemployed. Or a method in which the controls are executed in time seriesin order to prevent them from being overlapped with respect to time,that is, a method in which the above polarization-mode dispersioncontrolling step and the chromatic dispersion controlling step areexecuted in time series, may be employed.

Although the polarization-mode dispersion compensator and the chromaticdispersion compensator are controlled by the CPUs 239A and 239B, it isalternatively possible to use a control method by an analog circuitusing synchronous detection, not limited to the above example. It isalso possible to insert an A/D converter (not shown) and a D/A converter(not shown) in front of and behind each of the CPUs 239A and 239B.

Meanwhile, either one or both of the two different frequency componentsextracted in the electric stage may be used for extracting a timing of amain signal system.

(D3) Description of a Third Modification of the Third Embodiment

FIG. 59 is a block diagram of an optical transmission system accordingto a third modification of the third embodiment of this invention. Inthe optical transmission system 70C shown in FIG. 59, an opticaltransmitter 72C and an optical receiver 77C are connected over anoptical transmission path 73, and a dispersion compensation controllingapparatus 71C is disposed on the receiving side.

The optical transmitter 72C comprises a signal light source 8C and achromatic dispersion compensator 4C (denoted as Tx in FIG. 59) of achromatic dispersion compensation quantity variable type. As a chromaticdispersion equalizer, the signal light source 8C is configured with alaser diode of a variable wavelength or the like, and a signal opticalwavelength is optimized according to chromatic dispersion of atransmission line by the chromatic dispersion compensator 4C. Theoptical receiver 77C comprises a polarization-mode dispersioncompensator 74 of a polarization-mode dispersion compensation quantityvariable type and a photo receiving unit 76, thereby performing anoptimum value control at all times during system operation.Incidentally, parts in FIG. 59 designated by the same referencecharacters in FIG. 59 have the same or similar functions. In thismodification, a term “dispersion” is used to mean both“polarization-mode dispersion” and “chromatic dispersion”. When achromatic dispersion compensation quantity and a polarization-modedispersion compensation quantity are set to optimum values only at thetime of start of system operation, it is not necessarily that thecompensators are “variable”. For example, it is alternatively possibleto insert a “fixed” dispersion compensator such as a dispersioncompensating fiber, a dispersion compensator of a fiber grating type, orthe like.

As this control method, the dispersion compensation controllingapparatus 71C, using the control mode 2, feedback-controls a chromaticdispersion compensator 83 disposed in the optical transmission line 73such that the intensity of the second specific frequency componentdetected by a second intensity detecting unit (intensity detector 80B)becomes the maximum or the minimum. Since it is only necessary tocontrol the value to be the maximum value or the minimum value, it ispossible to perform the controls simultaneously and independently evenif they have dependency on each other. For example, in the case of 40Gb/s NRZ system, it is only necessary to control such that thef_(GVD)=40 GHz intensity becomes the minimum while the f_(PMD)=20 GHzintensity becomes the maximum value. It is alternatively possible tocontrol a monitor value to be an absolute value in the control mode 1.

As a method of switching the controls when a chromatic dispersioncompensation value and a polarization-mode dispersion compensation valueare controlled to be the optical values at all times during systemoperation, a method in which the above controls are executedindependently and in parallel with respect to time, that is, a method inwhich the above polarization-mode dispersion controlling step and thechromatic dispersion controlling step are executed independently, may beemployed. Or a method in which the controls are executed in time seriesin order to prevent them from being overlapped with respect to time,that is, a method in which the above polarization-mode dispersioncontrolling step and the chromatic dispersion controlling step areexecuted in time series, may be employed.

Although the polarization-mode dispersion compensator and the chromaticdispersion compensator are controlled by the CPUs 239A and 239B, it isalternatively possible to use a control method by an analog circuitusing synchronous detection or the like, not limited to the aboveexample. It is also possible to insert an A/D converter (not shown) anda D/A converter (not shown) in front of and behind each of the CPUs 239Aand 239B.

The other parts denoted by the same reference characters as thosedescribed above have the same or similar functions, further descriptionsof which are thus omitted. Here is used a simultaneous monitoring methodin a system corresponding to FIG. 57, but this invention is not limitedto this example. It is alternatively possible to use a simultaneousmonitoring method in a system corresponding to FIG. 53 or FIG. 55.

(D4) Description of a Fourth Modification of the Third Embodiment

FIG. 60 is a block diagram showing a structure of an opticaltransmission system to which a dispersion compensation controllingapparatus according to a fourth modification of the third embodiment ofthis invention is applied. This dispersion compensation controllingapparatus differs from the dispersion compensation controlling apparatusaccording to the third embodiment in that parameter information inputtedto a chromatic dispersion compensating unit 83 and a polarization-modedispersion compensating unit 74 are optimized.

Namely, in the optical transmission system 271A shown in FIG. 60, anoptical transmitter 72 as a transmitting terminal apparatus transmittinga transmission optical signal and an optical receiver 77 as a receivingterminal apparatus receiving the transmission optical signal areconnected over an optical transmission line (transmission fiber) 73.Here, the optical transmitter 72 and the optical transmitter 73 and theoptical receiver 77 are similar to those described above, furtherdescriptions of which are thus omitted. In this modification, a term“dispersion” is used to mean both “polarization-mode dispersion” and“chromatic dispersion”.

A dispersion compensation controlling apparatus 245 is disposed on thereceiving side. The dispersion compensation controlling apparatus 245comprises a photo receiver 78, band-pass filters 79A and 79B, intensitydetectors 80A and 80B, a parameter setting circuit 82 and a chromaticdispersion compensation quantity setting circuit 82B similar to those ofthe dispersion compensation controlling apparatus 71 according to thethird embodiment, along with compensation quantity optimizationcontrolling units 246 a and 246 b. The photo receiver 78, the band-passfilters 79A and 79B, the intensity detectors 80A and 80B, the parametersetting circuit 82 and the chromatic dispersion compensation quantitysetting circuit 82B have similar functions and structures to thoseaccording to the third embodiment, further descriptions of which arethus omitted. Therefore, the detection form 1 is employed.

The compensation quantity optimization controlling units 246 a and 246 bautomatically perform feedback controls when polarization-modedispersion and chromatic dispersion are compensated during systemoperation, in which the control mode 2 is used. The compensationquantity optimization controlling units 246 a and 246 b superimposepredetermined low frequency signals set in advance on a parametersetting control signal and a chromatic dispersion compensation quantitycontrol signal outputted from the parameter setting circuit 82 and thechromatic dispersion compensation quantity setting circuit 82B,respectively, and control parameter settings in the parameter settingcircuit 82 and the chromatic dispersion quantity setting circuit 82Bsuch that the above low frequency signal components included in theintensity of the above first specific frequency component from a firstintensity detecting unit (intensity detectors 80A and 80B) becomes zero,thereby optimizing a polarization-mode dispersion compensation quantityand a chromatic dispersion compensation quantity of the abovetransmission optical signal. The compensation quantity optimizationcontrolling units 246 a and 246 b comprises band-pass filters 272 a and272 b, phase comparing circuits 273 a and 273 b, low frequencyoscillators 274 a and 274 b, and low frequency superimposing circuits275 a and 275 b, respectively. The band-pass filters 272 a and 272 b,the phase comparing circuits 273 a and 273 b, the low frequencyoscillators 274 a and 274 b and the low frequency superimposing circuits275 a and 275 b are similar to the band-pass filter 32, the phasecomparing circuit 33, the low frequency oscillator 34 and the lowfrequency superimposing circuit 35 described in the fifth modificationof the first embodiment, respectively, further descriptions of which arethus omitted.

The compensation optimization controlling units 246 a and 246 b minutelymodulate a chromatic dispersion compensation quantity to be given by thechromatic dispersion compensator 83 and a delay quantity Δτ to be givenby the polarization-mode dispersion compnesator 74 with low frequency f₀(Hz) in order to automatically fix the intensity of the first specificfrequency component in a baseband spectrum of a transmission opticalsignal inputted to the receiving side over the optical transmission line73 to the maximum value. During system operation, the compensationoptimization controlling units 246 a and 246 b perform a trackingcontrol to keep a chromatic dispersion compensation quantity and apolarization-mode delay quantity Δτ at optimum values at all timesagainst a change with time of the optical transmission line 73. As anexample of this tracking control, a delay quantity Δτ is minutelychanged (dithered) in the vicinity of the maximum point Δτ₀ to detect anew maximum point, thereby automatically determining it in the feedbackcontrol when polarization-mode dispersion is compensated. In thefeedback control when chromatic dispersion is compensated, a chromaticdispersion compensation quantity is minutely changed in the vicinity ofthe maximum point to detect a new maximum point, thereby automaticallydetermining it. A method of the feedback control by the compensationquantity optimization controlling units 246 a and 246 b is similar tothat described above, further description of which is thus omitted.

It is alternatively possible to employ the control mode 1 in lieu of thecontrol mode 2. Although not shown, it is possible to provide apolarization-mode dispersion detecting unit and a chromatic dispersiondetecting unit for determining optimum values of parameter informationshowing a polarization-mode dispersion compensation quantity and achromatic dispersion compensation quantity before system operation, andswitches for switching outputs of the intensity detectors 80A and 80B,respectively.

With the above structure, in the optical transmission system 271A, anoptical signal at a transmission rate B (b/2) transmitted from theoptical transmitter 72 is transmitted to the optical receiver 77 overthe optical transmission line 73. In order to compensate chromaticdispersion and polarization-mode dispersion generated in the transmittedoptical signal, a part of the optical signal transmitted over theoptical transmission line 73 is taken out by an optical splitting unit75, and the optical signal taken out (monitor light) is sent to thedispersion compensation controlling apparatus 245. The optical signaltaken out by the optical splitting unit 75 is O/E-converted by the photoreceiver 78, split into two, and inputted to the band-pass filters 79Aand 79B. The first specific frequency component [B/2 (Hz) component] ina baseband spectrum is detected by the band-pass filter 79A, while thesecond specific frequency component [B (Hz) component] in the basebandspectrum is detected by the band-pass fitler 79B (specific frequencycomponents detecting step). Following that, intensities of the abovefirst specific frequency component and the second specific frequencycomponent detected by the band-pass filters 79A and 79B, respectively,are detected by the intensity detectors 80A and 80B (intensitiesdetecting step).

Following that, a parameter setting in the parameter setting circuit 82is such controlled by the compensation quantity optimization controllingunit 246 a that a low frequency signal component included in theintensity of the first specific frequency component from the intensitydetector 80A becomes zero, whereby a compensation quantity ofpolarization-mode dispersion of the above transmission optical signal isoptimized. A parameter setting control signal is outputted to thepolarization-mode dispersion compensator 74 disposed in the opticalreceiver 77 via the low frequency superimposing circuit 275 a in thecompensation quantity optimization controlling unit 246 a. When thepolarization-mode dispersion compensator 74 receives the parametersetting control signal, parameter information is set on the basis of thecontrol signal therein, whereby polarization-mode dispersion generatedin an optical signal transmitted over the optical transmission line 73is compensated. Incidentally, the parameter setting circuit 82 detects acode of a signal obtained as a result of phase comparison by the phasecomparing circuit 273 a, thereby determining whether a delay quantity Δτis shifted to a negative or a positive direction, so that a parametersetting control signal for changing the delay quantity Δτ in such adirection that the f₀ component intensity modulation component in theB/2 (Hz) component is generated and outputted. Further, the lowfrequency superimposing circuit 275 a superimposes a low frequencysignal (f₀ [Hz] signal) from the low frequency oscillator 274 a on theparameter setting control signal from the parameter setting circuit 82,and outputs it.

Similarly, the compensation quantity optimization controlling unit 246 bcontrols a chromatic dispersion compensation quantity in the chromaticdispersion compensation quantity setting circuit 82B such that a lowfrequency signal component included in the intensity of the secondspecific frequency component from the intensity detector 80B becomeszero, thereby optimizing a chromatic dispersion compensation quantity ofthe above transmission optical signal.

The dispersion compensation controlling apparatus 245 according to thefourth modification of the third embodiment of this invention detectsintensities of the first specific frequency component and the secondspecific frequency component in a baseband spectrum of a transmissionoptical signal, detects a polarization-mode dispersion quantity of thetransmission optical signal from the intensity of the first specificfrequency component by performing a predetermined first functionaloperation, thereby easily detecting polarization-mode dispersiongenerated in the transmission optical signal, while detecting achromatic dispersion compensation quantity of the transmission opticalsignal from the intensity of the second specific frequency component byperforming a predetermined second functional operation, thereby easilydetecting a chromatic dispersion quantity generated in the transmissionoptical signal.

As above, it is advantageous that a delay quantity Δτ can be at alltimes kept at the optimum value against a change with time of theoptical transmission line 23 during system operation, and deteriorationof the transmission waveform of an optical signal can be prevented bydetecting a polarization-mode dispersion quantity and a chromaticdispersion quantity, setting parameter information generated in thetransmission optical signal on the basis of the detected quantities andcompensating the polarization-mode dispersion and a chromatic dispersionquantity, which largely contributes to a long-distance transmission of ahigh-speed optical signal. Further, with the compensation quantityoptimization controlling units 246 a and 246 b, it is possible tooptimize compensation quantities of polarization-mode dispersion andchromatic dispersion quantity of a transmission optical signal, andautomatically perform a feedback control when polarization-modedispersion and chromatic dispersion are compensated.

(D5) Description of a Fifth Modification of the Third Embodiment

It is alternatively possible to extract a timing in the electric stage.FIG. 61 is a structure of an optical transmission system according to afifth modification of the third embodiment of this invention. Adispersion compensation controlling apparatus 70′ on the receiving sideshown in FIG. 61 comprises a timing extracting unit 84. Other partsdenoted by the same reference characters have the same or similarfunctions to those described above, further descriptions of which arethus omitted. Further, in this modification, a term “dispersion” is usedto mean both “polarization-mode dispersion” and “chromatic dispersion”,as well.

The timing extracting unit 84 extracts a timing of a received signal onthe basis of a specific frequency component detected by at least eitherband-pass filters 79A or 79B. On the basis of the specific frequencycomponents detected by these band-pass filters 79A and 79B, a timing ofthe received signal is extracted, and a clock signal taken out is sentto an optical receiving unit 76 of an optical receiver 77. In theoptical receiving unit 76 of the optical receiver 76, this clock signalis used for discrimination or the like.

With the above structure, frequency components of a received signalO/E-converted by a photo receiver 78 are detected by the band-passfilters 79A and 79B. Since these frequency components are signals insynchronization with a received waveform, a clock signal is taken out bythe timing extracting unit 84, inputted to the optical receiver 76 to beused for timing discrimination or the like in the main signal system. Anoptical transmission system 70′ to which a dispersion compensationcontrolling apparatus 71′ according to the fifth modification of thethird embodiment is applied operates in a similar manner to the opticaltransmission system 70 to which the dispersion compensation controllingapparatus 71 according to the above third embodiment is applied.

As above, according to the dispersion compensation controlling apparatus71′ according to the fifth modification of the third embodiment of thisinvention, it is possible to attain similar advantages to the case ofthe fourth embodiment described above. In addition, it is possible toimprove functions of the optical receiver 77 of the optical transmissionsystem 70′ by extracting a clock signal by the timing extracting unit84.

(E) Description of a Fourth Embodiment of This Invention

According to the third embodiment, there are obtained two systems offrequency numeral values to be monitored. However, it is alternativelypossible to unify them into one system, simultaneously monitor both achromatic dispersion value and a polarization-mode dispersion value of atransmission line, and simultaneously compensate transmission opticalwaveform deterioration due to the both. Incidentally, a term“dispersion” is used to mean both “polarization-mode dispersion” and“chromatic dispersion”, as well.

FIG. 62 is a block diagram of an optical transmission system accordingto a fourth embodiment of this invention. The optical transmissionsystem 270 shown in FIG. 62 is an optical communication system with atransmission rate B (b/s) (for example, 40 Gb/s, 10 Gb/s or the like)adopting time division multiplexing. The optical transmission system 270differs from the optical transmission system 70 according to the thirdembodiment in that a band-pass filter disposed at an output of a photoreceiver 78 is of one system. Other parts are almost similar to those ofthe optical transmission system 70 according to the third embodiment.

Namely, in the optical transmission system 270, an optical transmitter72 as a transmitting terminal apparatus transmitting a transmissionoptical signal and an optical receiver 77 as a receiving terminalapparatus receiving the transmission optical signal are connected overan optical transmission line (transmission fiber) 73, and a dispersioncompensation controlling apparatus 271 is disposed on the receivingside. The optical transmitter 72 and the optical transmission line 73are similar to those described above, further descriptions of which arethus omitted.

The optical receiver 77 comprises a chromatic dispersion compensator 83,a polarization-mode dispersion compensator 74, an optical splitting unit75 and an optical receiving unit 75. Similarly to the above thirdembodiment, both of the chromatic dispersion compensator 83 and thepolarization-mode dispersion compensator 74 are of variable type. Achromatic dispersion compensation quantity and a polarization-modedispersion compensation quantity can be at all timesoptimum-value-controlled in the control mode 2 during system operation.Incidentally, the optical splitting unit 75 and the optical receivingunit 75 are similar to those described above.

The dispersion compensation controlling apparatus 271 monitors a stateof polarization-mode dispersion and a state of chromatic dispersiongenerated in an optical signal transmitted over the optical transmissionline 73 on the basis of an optical signal taken out by the opticalsplitting unit 75 in the optical receiver 77, and controls thepolarization-mode dispersion compensator 74 and the chromatic dispersioncompensator 83 according to results of the monitoring. The dispersioncompensation controlling apparatus 271 comprises a photo receiver 78, aband-pass filter (fe BPF) 79, an intensity detector 80, apolarization-mode dispersion quantity-chromatic dispersion quantitydetecting unit 81C, a chromatic dispersion compensation quantity settingcircuit 82B and a parameter setting circuit 82. The photo receiver 78,the band-pass filter 79 and the intensity detector 80 are similar tothose described in the third embodiment.

The polarization-mode dispersion quantity-chromatic dispersion quantitydetecting unit 81C detects the above polarization-mode dispersionquantity and the chromatic dispersion quantity on the basis of afrequency of the first specific frequency component detected by theintensity detector 80. Incidentally, the first specific frequencycomponent is appropriately set according to a transmission rate or asignal waveform of an optical signal. With respect to this frequency,when above transmission optical signal is an RZ optical signal or anoptical time division multiplex signal, a first specific frequencycomponent detecting unit (polarization-mode dispersionquantity-chromatic dispersion quantity detecting unit 81C) detects afrequency corresponding to the bit rate or ½ of the bit rate as thefirst specific frequency component. When the above transmission opticalsignal is an NRZ signal, the first specific frequency componentdetecting unit (polarization-mode dispersion quantity-chromaticdispersion quantity detecting unit 81C) detects a frequencycorresponding to ½ of the bit rate as the first specific frequencycomponent.

The chromatic dispersion compensation quantity setting circuit 82Bfeedback-controls the chromatic dispersion compensator 83 disposed inthe optical transmission line 73 such that the intensity of the firstspecific frequency component detected by the polarization-modedispersion quantity-chromatic dispersion quantity detecting unit 81Cbecomes the maximum or the minimum. The polarization-mode dispersionquantity-chromatic dispersion quantity detecting unit 81C and thechromatic dispersion compensation quantity setting circuit 82B functionas a chromatic dispersion controlling unit 241 a.

Namely, the chromatic dispersion controlling unit 241 afeedback-controls the chromatic dispersion comensator 83 disposed in theoptical transmission line 73 such that the intensity of the firstspecific frequency component detected by the intensity detector 80becomes the maximum or the minimum, in other words, the control mode 2is employed.

The parameter setting circuit 82 outputs a parameter setting controlsignal having parameter information as a control quantity forcompensating polarization-mode dispersion of the transmission opticalsignal on the basis of a polarization-mode dispersion quantity detectedby the polarization-mode dispersion quantity-chromatic dispersionquantity detecting unit 81C to the polarization-mode dispersioncompensator 74 in the optical receiver 77. The polarization-modedispersion quantity-chromatic dispersion quantity detecting unit 81C andthe parameter setting circuit 82 function as the polarization-modedispersion controlling unit 241 b.

From the above, the dispersion compensation controlling apparatus 271 isconfigured with the first specific frequency detecting unit (band-passfilter 79) detecting the first specific frequency component in abaseband spectrum in a transmission optical signal inputted to thereceiving side over a transmission fiber as a transmission line, a firstintensity detecting unit (intensity detector 80) detecting informationon an intensity of the above first specific frequency component detectedby the first specific frequency component detecting unit (band-passfilter 79), a polarization-mode dispersion controlling unit(polarization-mode dispersion quantity-chromatic dispersion quantitydetecting unit 81C and parameter setting circuit 82) controlling apolarization-mode dispersion quantity of the transmission line (opticaltransmission line 73) such that the intensity of the first specificfrequency component detected by the first intensity detecting unit(intensity detector 80) becomes the maximum, and a chromatic dispersioncontrolling unit (polarization-mode dispersion quantity-chromaticdispersion quantity detecting unit 81C and chromatic dispersioncompensation quantity setting circuit 82B) controlling a chromaticdispersion quantity of the transmission line (optical transmission line73) such that the intensity of the first specific frequency componentdetected by the first intensity detecting unit (intensity detector 80)becomes the maximum.

A flow of a signal in the dispersion compensation controlling apparatus271 according to the fourth embodiment shown in FIG. 62 is as follows.

An optical signal taken out by the optical splitting unit 75 is firstreceived by the photo receiver 78, O/E-converted into an electricsignal, inputted to the band-pass filter 79, the first specificfrequency component [fe (Hz) component] in a baseband spectrum in thetransmission optical signal is detected by the band-pass filter 79, andan intensity of the above first specific frequency component detected bythe band-pass filter 79 is detected by the intensity detector 80.

A polarization-mode dispersion quantity and a chromatic dispersionquantity of the above transmission optical signal are detected by thepolarization-mode dispersion quantity-chromatic dispersion quantitydetecting unit 81C such that the intensity of the first specificfrequency component detected by the intensity detector 80 becomes themaximum, a parameter setting control signal for setting such parameterinformation (delay quantity Δτ and optical intensity splitting ratio γ)as to cancel the polarization-mode dispersion quantity detected by thepolarization-mode dispersion quantity-chromatic dispersion quantitydetecting unit 81C is outputted from the parameter setting circuit 82 tothe polarization-mode dispersion compensator 74 disposed in the opticalreceiver 77 in order to compensate polarization-mode dispersion of thetransmission optical signal, and a control signal for setting achromatic dispersion control quantity is outputted from the chromaticdispersion compensation quantity setting circuit 82B to the chromaticdispersion compensator 83 disposed in the optical transmission line 73on the basis of the above chromatic dispersion quantity detected by thepolarization-mode dispersion quantity-chromatic dispersion quantitydetecting unit 81C in order to compensate chromatic dispersion of theabove transmission optical signal. In the case of the 40 Gb/s NRZsystem, for example, the f_(GVD)=40 GHz intensity is such controlled asto be the minimum while the F_(PMD)=20 GHz intensity is such controlledas to be the maximum.

When the polarization-mode dispersion compensator 74 receives theparameter setting control signal, parameter information is set on thebasis of the control signal therein, whereby polarization-modedispersion generated in an optical signal transmitted over the opticaltransmission line 73 is compensated. When the chromatic dispersioncompensator 83 receives the control signal, chromatic dispersiongenerated in the optical signal transmitted over the opticaltransmission line 73 is compensated on the basis of the control signal.

As above, the intensity can be controlled to be the maximum or theminimum, and the controls can be performed simultaneously andindependently.

Meanwhile, a method of controlling the chromatic dispersion compensator83 and the polarization-mode dispersion compensator 74 may be in thecontrol mode 1, in which the monitor values can be controlled to beabsolute values. Namely, the chromatic dispersion controlling unit 241 amay set a chromatic dispersion control quantity in the chromaticdispersion compensator 83 disposed in the optical transmission line 73such that the intensity of the first specific frequency componentdetected by the first intensity detecting unit (intensity detector 80)becomes the maximum or the minimum. In such case, the polarization-modedispersion quantity-chromatic dispersion quantity detecting unit 81Cdetects a chromatic dispersion quantity of the above transmissionoptical signal from the intensity of the above first specific frequencycomponent detected by the first intensity detecting unit (intensitydetector 80) by performing a predetermined operation with apredetermined second function, detects a polarization-mode dispersionquantity of the above transmission optical signal from the intensity ofthe first specific frequency component detected by the intensitydetector 80 by performing a predetermined first functional operation,the chromatic dispersion compensation quantity setting circuit 82B setsa chromatic dispersion control quantity in the chromatic dispersioncompensator 83 on the basis of the above chromatic dispersion quantitydetected by the chromatic dispersion quantity detecting unit(polarization-mode dispersion quantity-chromatic dispersion quantitydetecting unit 81C) in order to compensate chromatic dispersion in theabove transmission optical signal.

The dispersion compensation controlling apparatus 271 may outputinformation on the intensity of the above first specific frequencycomponent detected by the intensity detector 80 shown in FIG. 62 as amonitor signal.

When a chromatic dispersion quantity and a polarization-mode dispersionquantity are set to the optimum values only at the time of start ofsystem operation, the compensators are not necessarily “variable”. Forexample, it is possible to insert a “fixed” dispersion compensator suchas a dispersion compensating fiber, a dispersion compensator of a fibergrating type, or the like.

As a method of switching the controls in the case where a chromaticdispersion compensation quantity and a polarization-mode dispersioncompensation quantity are controlled to be the optimum values at alltimes during system operation, a method in which the above controls areperformed in parallel with respect to time, that is, a method in whichthe above polarization-mode dispersion controlling step and thechromatic dispersion controlling step are performed independently, maybe employed. Or a method in which the controls are performed in timeseries in order to prevent them from being overlapped with respect totime, that is, a method in which the above polarization-mode dispersioncontrolling step and the chromatic dispersion controlling step areexecuted in time series, may be employed.

Further, the polarization-mode dispersion compensator and the chromaticdispersion compensator are controlled by CPUs 239A and 239B. However,this embodiment is not limited to the above example. A control method byan analog circuit using synchronous detection or the like may beemployed. It is also possible to insert an A/D converter (not shown) anda D/A converter (not shown) in front of and behind each of the CPUs 239Aand 230B.

The optical transmission system 270 to which the dispersion compensationcontrolling apparatus 271 according to the fourth embodiment with theabove structure operates in a similar manner to the optical transmissionsystem 70 to which the dispersion compensation controlling apparatus 71according to the above third embodiment described above. Namely, thisdispersion compensation controlling step comprises a step of detectingthe first specific frequency component in a baseband spectrum in atransmission optical signal inputted to the receiving side over atransmission fiber as a transmission line (first specific frequencycomponent detecting step) a step of detecting information on anintensity of the first specific frequency component detected at thefirst specific frequency detecting step (first intensity detectingstep), a step of controlling a polarization-mode dispersion quantity ofthe optical transmission line 73 such that the intensity of the firstspecific frequency component detected at the first intensity detectingstep becomes the maximum (polarization-mode dispersion controllingstep), and a step of controlling a chromatic dispersion quantity of theoptical transmission line 73 such that the intensity of the firstspecific frequency component detected at the first intensity detectingstep becomes the maximum or the minimum (chromatic dispersioncontrolling step).

As above, according to the dispersion compensation controlling apparatus271 of the fourth embodiment of this invention, it is possible to attainthe similar advantages to the case of the third embodiment describedabove.

(E1) Description of a First Modification of the Fourth Embodiment

In the fourth embodiment, either one or both of the two differentfrequency components extracted in the electric stage may be used toextract a timing for the main signal system. FIG. 63 is a block diagramof an optical transmission system according to a first modification ofthe fourth embodiment of this invention. A dispersion compensationcontrolling apparatus 70A shown in FIG. 63 is of a structure in the casewhere a timing extracting unit 84 is provided. A signal from the timingextracting unit 84 is inputted to an optical receiving unit 6 to timethe main signal system.

The optical transmission system 70A shown in FIG. 63 is an opticalcommunication system with a transmission rate B (b/s) (for example, 40Gb/s, 10 Gb/s, or the like) adopting time division multiplexing. Theoptical transmission system 70A differs from the optical transmissionsystem 270 according to the fourth embodiment in that the timingextracting unit 84 is provided, but the other parts are similar to thoseof the optical transmission system 270 according to the fourthembodiment. In this modification, a term “dispersion” is used to meanboth “polarization-mode dispersion” and “chromatic dispersion”.

Namely, a dispersion compensation controlling apparatus 70A monitors astate of polarization-mode dispersion and a state of chromaticdispersion generated in an optical signal transmitted over an opticaltransmission line 73 on the basis of an optical signal taken out by anoptical splitting unit 75, and controls a polarization-mode dispersioncompensator 74 and a chromatic dispersion compensator 83. The dispersioncompensation controlling apparatus 71A comprises, as shown in FIG. 62, aphoto receiver 78, a band-pass filter (fe BPF) 79, an intensity detector80, a polarization-mode dispersion quantity-chromatic dispersionquantity detecting unit 81C, a parameter setting circuit 82, a chromaticdispersion compensation quantity setting circuit 82B and the timingextracting unit 84.

The photo receiver 78, the intensity detector 80, the parameter settingcircuit 82 and the chromatic dispersion compensation quantity settingcircuit 82B have similar functions and structures to those describedabove in the fourth embodiment. The detection form 1 is employed.

The band-pass filter 79 detects the first specific frequency component[fe (Hz) component] in a baseband spectrum of a transmission opticalsignal inputted to the receiving side over the optical transmission line73. Incidentally, the first specific frequency component isappropriately set according to a transmission rate or a signal waveformof an optical signal. In the optical transmission system 70A shown inFIG. 63, a frequency of the first specific frequency component used whenthe above polarization-mode dispersion quantity and the chromaticdispersion quantity are detected by the polarization-mode dispersionquantity-chromatic dispersion quantity detecting unit 81C is set to afrequency corresponding to the bit rate.

Further, the polarization-mode dispersion quantity-chromatic dispersionquantity detecting unit 81C has a function as the polarization-modedispersion quantity detecting unit 81 and the chromatic dispersionquantity detecting unit 81B according to the fourth embodiment describedabove.

Namely, the dispersion compensation controlling apparatus 71A isconfigured with a chromatic dispersion quantity detecting unit detectinga chromatic dispersion quantity of a transmission optical signal from anintensity of the above first specific frequency component detected bythe intensity detector 80 by performing the predetermined secondfunctional operation described above, and a chromatic dispersioncompensation quantity setting circuit 82B setting a chromatic dispersioncontrol quantity in the chromatic dispersion compensator 83 disposed inthe optical transmission line 73 on the basis of the chromaticdispersion quantity detected by the chromatic dispersion quantitydetecting unit in order to compensate chromatic dispersion of thetransmission optical signal.

Meanwhile, a method of controlling the chromatic dispersion compensator83 and the polarization-mode dispersion compensator 74 may be in thecontrol mode 1. It is possible to insert an A/D converter (not shown)and a D/A converter (not shown) in front of and behind a CPU, therebyusing a control method by an analog circuit using synchronous detectionor the like. When a chromatic dispersion compensation quantity and apolarization-mode dispersion compensation quantity are set to theoptimum values only at the time of start of system operation, thecompensators are not necessarily “variable”. For example, a “fixed”dispersion compensator such as a dispersion compensating fiber, adispersion compensator of a fiber grating type or the like may beinserted. As a method of switching the controls in the case where achromatic dispersion compensation quantity and a polarization-modedispersion compenation quantity are controlled to be the optimum valuesat all times during system operation, a method in which the abovecontrols are executed independently and in parallel with respect totime, that is, the above polarization-mode dispersion controlling stepand the chromatic dispersion controlling step are executedindependently, maybe employed. Or, a method in which the controls areexecuted in time series in order to prevent them from being overlapped,that is, a method in which the above polarization-mode dispersioncontrolling step and the chromatic dispersion controlling steps areexecuted in time series, may be employed.

The timing extracting unit 84 extracts a timing of a received signal onthe basis of the specific frequency component detected by the band-passfilter 79. As the timing extracting unit 84, PLL or the like is used. Atiming of a received signal is extracted on the basis of the specificfrequency component detected by the band-pass filter 79, and the clocksignal taken out is sent to an optical receiving unit 76 of the opticalreceiver 77. In the optical receiving unit 76 of the optical receiver76, this clock signal is used for discrimination or the like.

Namely, since the fe (Hz) component is a signal in synchronization witha received waveform, a clock signal can be taken out by the timingextracting unit 84, and used for discrimination in the optical receiver76.

With the above structure, the optical transmission system 70A to whichthe dispersion compensation controlling apparatus 71A according to thefirst modification of the fourth embodiment operates in almost a similarmanner to the optical transmission system 270 to which the dispersioncompensation controlling apparatus 271 according to the fourthembodiment described above is applied.

As above, according to the dispersion compensation controlling apparatus71A according to the first modification of the fourth embodiment of thisinvention, it is possible to attain the similar advantages to the fourthembodiment described above. It is also possible to improve functions ofthe optical receiver 77 of the optical transmission system 70A byextracting a clock signal by the timing extracting unit 84.

(E2) Description of a Second Modification of the Fourth Embodiment

FIG. 64 is a block diagram of an optical transmission system accordingto a second modification of the fourth embodiment of this invention,which differs in that the band-pass filer is in one system. Namely,unlike the dispersion compensation controlling apparatus 245 in FIG. 60corresponding to the fourth modification of the third embodiment, anoutput of the photo receiver 78 is outputted only to the band-passfilter [fe BPF] 79.

Namely, the optical transmission system 271B shown in FIG. 64 comprisesan optical transmitter 72, an optical receiver 77 and an opticaltransmission line (transmission fiber) 73 along with a dispersioncompensation controlling apparatus 247. The dispersion compensationcontrolling apparatus 247 comprises a photo receiver 78, a band-passfilter [fe BPF] 79, an intensity detector 80, a parameter settingcircuit 82, a chromatic dispersion compensation quantity setting circuit82B and compensation quantity optimization controlling units 246 a and246 b. The photo receiver 78, the band-pass filter 79, the intensitydetector 80, the parameter setting circuit 82, the chromatic dispersioncompensation quantity setting circuit 82B, the parameter setting circuit82, the chromatic dispersion compensation quantity setting circuit 82Band the compensation quantity optimization controlling units 246 a and246 b have similar functions and structures to those described above,further descriptions of which are thus omitted. The compensationquantity optimization controlling units 246 a and 246 b compriseband-pass filters 272 a and 272 b, phase comparing circuits 273 a and273 b, low frequency oscillators 274 a and 274 b and low frequencysuperimposing circuits 275 a and 275 b, respectively. The band-passfilters 272 a and 272 b, the phase comparing circuits 273 a and 273 b,the low frequency oscillators 274 a and 274 b, and the low frequencysuperimposing circuits 275 a and 275 b are similar to those describedabove, further descriptions of which are thus omitted.

In this modification, a term “dispersion” is used to mean both“polarization-mode dispersion” and “chromatic dispersion”, as well.

As not shown, the optical transmission system 271B may further comprisea polarization-mode dispersion quantity detecting unit and a chromaticdispersion detecting unit for determining optimum values of parameterinformation showing a polarization-mode dispersion compensation quantityand a chromatic dispersion compensation quantity before systemoperation, and a switch for switching an output of the intensitydetector 80.

It is also possible to use a control method by an analog circuit usingsynchronous detection by inserting an A/D converter (not shown) and aD/A converter (not shown) in front of and behind a CPU. When a chromaticdispersion compensation quantity and a polarization-mode dispersioncompensation quantity are set to the optimum values only at the time ofstart of system operation, the compensators are not necessarily“variable”. For example, a “fixed” dispersion compensator such as adispersion compensating fiber, a dispersion compensator of a fibergrating type or the like may be inserted.

Although this control method uses the control mode 2, the control mode 1may be used. As a method of switching the controls in the case where achromatic dispersion compensation quantity and a polarization-modedispersion compensation quantity are controlled to be the optimum valuesat all times during system operation, a method in which the abovecontrols are executed in parallel with respect to time, that is, amethod in which the above polarization-mode dispersion controlling stepand the chromatic dispersion controlling step are executedindependently, maybe employed. Or, a method in which the controls areexecuted in time series in order to prevent them from being overlappedwith respect to time, that is, a method in which the abovepolarization-mode dispersion controlling step and the chromaticdispersion controlling step are executed in time series, may beemployed.

In the optical transmission system 271B with the above structure, anoptical signal at a transmission rate B (b/s) transmitted from theoptical transmitter 72 is transmitted to the optical receiver 77 overthe optical transmission line 73. In order to compensate chromaticdispersion and polarization-mode dispersion generated in a transmittedoptical signal, a part of the optical signal transmitted over theoptical transmission line 73 is taken out by an optical splitting unit75, and the optical signal taken out (monitor light) is sent to thedispersion compensation controlling apparatus 247. The optical signaltaken out by the optical splitting unit 75 is O/E-converted by the photoreceiver 78, and inputted to the band-pass filter 79. In the band-passfilter 79, the first specific frequency component [fe (Hz) component] inthe baseband spectrum is detected (specific frequency componentdetecting step). Following that, an intensity of the above firstspecific frequency component detected by the band-pass filter 79 isdetected by the intensity detector 80 (intensity detecting step).

Thereafter, in the compensation quantity optimization controlling unit246 a, parameter setting in the parameter setting circuit 82 is suchcontrolled that a low frequency signal component included in theintensity of the first specific frequency component from the intensitydetector 80 becomes zero, whereby a compensation quantity ofpolarization-mode dispersion of the above transmission optical signal isoptimized. A parameter setting control signal is outputted to thepolarization-mode dispersion compensator 74 disposed in the opticalreceiver 77 via the low frequency superimposing circuit 275 a of thecompensation quantity optimization controlling unit 246 a. When thepolarization-mode dispersion compensator 74 receives the parametersetting signal, parameter information is set therein on the basis of thecontrol signal, whereby polarization-mode dispersion generated in anoptical signal transmitted over the optical transmission path 73 iscompensated. The parameter setting circuit 82 detects a code of a signalobtained as a result of phase comparison by the phase comparing circuit273 a to determine whether a delay quantity Δτ is shifted to thepositive or negative direction, so that a parameter setting controlsignal for changing the delay quantity Δτ in such a direction that thef₀ (Hz) intensity modulation component in the B/2 (Hz) component iscancelled is generated, and outputted. The low frequency superimposingcircuit 275 a superimposes a low frequency signal (f₀ (Hz) signal) fromthe low frequency oscillator 274 a on the parameter setting controlsignal from the parameter setting circuit 82, and output it.

Similarly, the compensation quantity optimization controlling unit 246 bcontrols a chromatic dispersion compensation quantity in the chromaticdispersion compensation quantity setting circuit 82B such that a lowfrequency signal component included in the intensity of the firstspecific frequency component from the intensity detector 80 becomeszero, as well, whereby a chromatic dispersion compensation quantity ofthe above transmission optical signal is optimized.

According to the dispersion compensation controlling apparatus 247 ofthe second modification of the fourth embodiment of this invention, itis possible to detect an intensity of the first specific frequencycomponent in a baseband spectrum of a transmission optical signal anddetect a polarization-mode dispersion quantity of the transmissionoptical signal from the detected intensity of the first specificfrequency by performing a predetermined first functional operation,thereby easily detecting polarization-mode dispersion generated in thetransmission optical signal.

As above, it is possible to keep a delay quantity Δτ at the optimumvalue against a change with time of the optical transmission line 73during system operation. It is also possible to prevent deterioration ofa transmission waveform of an optical signal by detecting apolarization-mode dispersion quantity and a chromatic dispersionquantity, setting parameter information generated in a transmissionoptical signal on the basis of the detected quantities to compensatepolarization-mode dispersion and chromatic dispersion. Theseadvantageously contribute to long-distance transmission of a high-speedoptical signal. With the compensation quantity optimization controllingunits 246 a and 246 b, it is possible to optimize compensationquantities of polarization-mode dispersion and chromatic dispersion, andautomatically perform a feedback control when polarization-modedispersion and chromatic dispersion are compensated.

(F) Others

In the controlling methods according to the third embodiment, themodifications of the third embodiment, the fourth modification and themodifications of the fourth embodiment, polarization-mode dispersion andchromatic dispersion are both controlled in the same control mode 2 orthe control mode 1. However, it is alternatively possible to mix themand perform the control. Namely, the polarization-mode dispersioncontrol may be performed in the control mode 1, while the chromaticdispersion control may be performed in the control mode 2. Conversely,the polarization-mode dispersion control may be performed in the controlmode 2, while the chromatic dispersion control may be performed in thecontrol mode 1. The control method performed at the time of thecompensation optimizing control may be performed in the control mode 1.Further, in the third embodiment and the modifications thereof, and thefourth embodiment and the modifications thereof, a position of thechromatic dispersion compensator 83 and a position of thepolarization-mode dispersion compensator 74 are exchangeable.

Still further, in the polarization-mode dispersion control in the secondembodiment, the control on γ and Δτ_(c), and the control on α·β andΔτ_(c) may be performed in a mixture of the two kinds of the controlmodes, as well. In order to find a predetermined control value, it ispossible to combine the control mode 1 and the control mode 2. First, avalue in the vicinity of a predetermined control value may be roughlyobtained in the control mode 1, after that, an extreme value may besearched in the vicinity thereof in the control mode 2. These are shownin (1) and (2) below.

(1) Control on γ and Δτ_(c)

In the first embodiment, an optical intensity split light γ can becontrolled on only the receiving side. A structure being capable of suchcontrol is shown in FIGS. 19, 21 and 27 through 30. In these structures,control values for γ and Δτ_(c) are obtained in the control mode 1.However, it is alternatively possible to obtain control values for γ inthe control mode 2 and Δτ_(c) in the control mode 2, or control valuesfor γ in the control mode 2 and Δτ_(c) in the control mode 1, or controlvalues for both γ and Δτ_(c) in the control mode 2. Still alternatively,a control mode in which a value in the vicinity of a predeterminedcontrol value is roughly obtained in the control mode 1, an extremevalue is then searched in the vicinity thereof in the control mode 2 ispossible.

(2) Control on α, β and Δτ_(c)

Similarly, in the control in the second embodiment, control values forα, β and Δτ_(c) are obtained in the control mode 2. However, it ispossible to obtain α and β still in the control mode 2, while Δτ_(c) inthe control mode 1. It is alternatively possible to employ such acontrol mode that a value in the vicinity of a predetermined controlvalue is roughly obtained in the control mode 1, after that, an extremevalue is searched in the vicinity thereof in the control mode 2.

As detection frequency values in each of the embodiments andmodifications, fe=B (Hz) is used for an RZ signal and an OTDM signal,while fe=B/2 (Hz) for an NRZ signal. As these frequency values, it ispossible to set another frequency so long as a component in a basebandspectrum in a transmission optical signal is stably obtained withrespect to time as the first specific frequency component in a basebandspectrum in a transmission optical signal extracted by the band-passfilter.

Further, when the specific frequency is set to a frequency correspondingto ½ of the bit rate, the transmission optical signal in each of theembodiments and modifications described above may be applied anymodulation system including an NRZ signal, an RZ optical signal, opticaltime division multiplex signal.

As the delay quantity compensator described in the first embodiment, adelay quantity compensator 4A′ shown in FIG. 65 may be used, other thanone shown in FIG. 5. The delay quantity compensator 4A′ is a delayquantity compensator whose delay quantity is variable. As shown in FIG.65, the delay quantity compensator 4A′ comprises a polarizationcontroller 4A-2, polarization beam splitters (PBS: Polarization BeamSplitter) 4A-5 and 4A-6 and a variable optical delay 4A-7.

The polarization controller 4A-2 such controls that polarization-modeprimary axis component of two transmission paths are TE and TM polarizedwaves, which comprises a ¼ wave plate (λ/4 plate) 4A-21, a ½ wave plate(λ/2 plate) 4A-22 and actuators 4A-23 and 4A-24. The polarization beamsplitter 4A-5 splits an optical signal inputted via the polarizationcontroller 4A-2 into two. The variable optical delay 4A-7 variably givesa delay difference to one of the optical components split by thepolarization beam splitter 4A-5. The polarization beam splitter 4A-6multiplexes an optical component from the polarization beam splitter4A-5 and an optical component from the polarization beam splitter 4A-7.

The actuators 4A-23 and 4A-24 configuring the polarization controller4!-2, and the variable optical delay 4A-7 receive parameter settingcontrol signals from the parameter setting circuit 15. The optimalcontrol as the delay quantity compensator 4A′ is performed on apolarization direction in the polarization controller 4A-2 and a delaydifference to be given by the variable optical delay 4A-7.

In each of the embodiments described above, the polarization-modedispersion compensator or the chromatic dispersion compensator isdisposed in the optical transmitter or the optical receiver. However,this invention is not limited to the above examples, but they may bedisposed in a repeating apparatus repeating a transmission opticalsignal. In such case, the parameter setting circuit or the chromaticdispersion compensation quantity setting circuit outputs each controlsignal to the polarization-mode dispersion compensator or the chromaticdispersion compensator disposed in the above repeating apparatus.

INDUSTRIAL APPLICABILITY

As having been fully described, the polarization-mode dispersionquantity detecting method of this invention has an advantage that anintensity of a specific frequency component in a baseband spectrum in atransmission optical signal is detected, and a polarization-modedispersion quantity of the transmission optical signal is detected fromthe detected intensity of the specific frequency component by performinga predetermined functional operation or in a maximum value control,thereby easily detecting polarization-mode dispersion generated in thetransmission optical signal.

According to this invention, a polarization-mode dispersion quantity isdetected and polarization-mode dispersion generated in a transmissionoptical signal is compensated on the basis of the detectedpolarization-mode dispersion quantity, whereby deterioration of atransmission waveform of an optical signal is prevented. Thiscontributes to realization of long-distance transmission of a high-speedoptical signal.

According to this invention, a polarization-mode dispersion quantity isdetected, polarization-mode dispersion generated in a transmissionoptical signal is compensated on the basis of the detectedpolarization-mode dispersion quantity, a chromatic dispersion quantityis also detected, and chromatic dispersion generated in the transmissionoptical signal is compensated on the basis of the detected chromaticdispersion quantity, whereby deterioration of a transmission waveform ofan optical signal due to polarization-mode dispersion and chromaticdispersion is prevented. This contributes to realization oflong-distance transmission of a high-speed optical signal.

What is claimed is:
 1. A polarization-mode dispersion quantity detectingmethod, comprising: detecting a specific frequency component in abaseband spectrum in a transmission optical signal input over atransmission optical fiber, the specific frequency componentcorresponding to a bit rate; detecting an intensity of the specificfrequency component detected at said detecting a specific frequencycomponent; and detecting a polarization-mode dispersion quantity of thetransmission optical signal from information on the intensity of thespecific frequency component detected at said detecting an intensity byperforming a predetermined functional operation, wherein when thetransmission optical signal is an RZ optical signal or an optical timedivision multiplex signal, the specific frequency at which the componentis detected at said detecting a specific frequency component is set to afrequency corresponding to the bit rate.
 2. The polarization-modedispersion quantity detecting method according to claim 1, wherein thepredetermined functional operation is performed at said detecting apolarization-mode dispersion quantity using a function representing anintensity of a frequency component in a baseband spectrum in an opticalwaveform forming an arbitrary transmission optical signal and in whichthe frequency information and parameters showing a polarization-modedispersion quantity are variables.
 3. The polarization-mode dispersionquantity detecting method according to claim 1, wherein the specificfrequency at which the component is detected at said detecting aspecific frequency component is set to a frequency at which a componentin a baseband spectrum in the transmission optical signal can be stablyobtained with respect to time.
 4. The polarization-mode dispersionquantity detecting method according to claim 3, wherein when thetransmission optical signal is in any optical modulation system, thespecific frequency at which the component is detected at said detectinga specific frequency component is set to a frequency corresponding toone-half of the bit rate.
 5. A dispersion compensation controllingmethod comprising: detecting a first specific frequency component, thespecific frequency component corresponding to a bit rate, in a basebandspectrum in a transmission optical signal input to a receiving side overa transmission fiber as a transmission line; detecting information on anintensity of the first specific frequency component detected at saiddetecting a first specific frequency component; and controlling apolarization-mode dispersion quantity of the transmission line, with theintensity of the first specific frequency component, detected at saiddetecting information on an intensity, becoming the maximum, whereinwhen the transmission optical signal is an RZ optical signal or anoptical time division multiplex signal, the specific frequency at whichthe component is detected at said detecting a first specific frequencycomponent is set to a frequency corresponding to the bit rate.
 6. Adispersion compensation controlling apparatus, comprising: a firstspecific frequency component detecting unit detecting a first specificfrequency component, the specific frequency component corresponding to abit rate in a baseband spectrum in a transmission optical signal inputto a receiving side over a transmission fiber as a transmission line; afirst intensity detecting unit detecting information on an intensity ofthe first specific frequency component detected by said first specificfrequency component detecting unit; and a polarization-mode dispersioncontrolling unit controlling a polarization-mode dispersion quantity ofthe transmission line with the intensity of the first specific frequencycomponent detected by said first intensity detecting unit becoming themaximum, wherein when the transmission optical signal is an RZ signal oran optical time division multiplex signal, said first specific frequencycomponent detecting unit detects a frequency corresponding to the bitrate as the first specific frequency component.
 7. The dispersioncompensation controlling apparatus according to claim 6, wherein whenthe transmission optical signal is in any optical modulation system,said first specific frequency component detecting unit detects afrequency corresponding to one-half of the bit rate as the firstspecific frequency component.
 8. The dispersion compensation controllingapparatus according to claim 6, wherein said polarization-modedispersion controlling unit sets a polarization-mode dispersion controlquantity in a polarization-mode dispersion compensator disposed in thetransmission line with the intensity of the first specific frequencycomponent detected by said first intensity detecting unit becoming themaximum.
 9. The dispersion compensation controlling apparatus accordingto claim 6, wherein said polarization-mode dispersion controlling unitfeedback-controls at least either a polarization controller or aninter-polarization-mode delay unit disposed in said transmission linepath with the intensity of said first specific frequency componentdetected by said first intensity detecting unit becoming the maximum.10. The dispersion compensation controlling apparatus according to claim6, wherein said first intensity detecting unit outputs information onthe detected intensity of the first specific frequency component as amonitor signal.
 11. A dispersion compensation controlling apparatus,comprising: a first specific frequency component detecting unitdetecting a first specific frequency component in a baseband spectrum ina transmission optical signal input to a receiving side over atransmission fiber as a transmission line; a first intensity detectingunit detecting information on an intensity of the first specificfrequency component detected by said first specific frequency componentdetecting unit; and a polarization-mode dispersion controlling unitcontrolling a polarization-mode dispersion quantity of the transmissionline with the intensity of the first specific frequency componentdetected by said first intensity detecting unit becoming the maximum,and performing a feedback control on an inter-polarization-mode variabledelay unit disposed in the transmission line, saidinter-polarization-mode variable delay unit giving a delay differencebetween polarization modes to perform variable polarization-modedispersion compensation, wherein when the transmission optical signal isan RZ signal or an optical time division multiplex signal, said firstspecific frequency component detecting unit detects a frequencycorresponding to the bit rate as the first specific frequency component.12. A polarization-mode dispersion quantity detecting method,comprising: detecting a specific frequency component in a basebandspectrum in a transmission optical signal input over a transmissionoptical fiber, the specific frequency component corresponding to a bitrate; detecting an intensity of the specific frequency componentdetected at said detecting a specific frequency component; and detectinga polarization-mode dispersion quantity of the transmission opticalsignal from information on the intensity of the specific frequencycomponent detected at said detecting an intensity by performing apredetermined functional operation, wherein the specific frequency atwhich the component is detected at said detecting a specific frequencycomponent is set to a frequency at which a component in a basebandspectrum in the transmission optical signal can be stably obtained withrespect to time, and wherein when the transmission optical signal is anRZ optical signal or an optical time division multiplex signal, thespecific frequency at which the component is detected at said detectinga specific frequency component is set to a frequency corresponding tothe bit rate.